The mountains did it.

What happened to Namdaemun?[photo from www.koreatimes.co.kr]
ONE theory, according to Jeon Hang-soo, head of Korea Oriental Topography Research Center, is that the gate was built to block the fire energy emitted from the pointy mountains around the city, and that if the gate weren't there, Gyeongbok Palace, a ways behind it, would have caught fire.

Mysterious Energy Linked to Blaze
By Park Si-soo Staff Reporter 02-11-2008 17:49
Oriental topography experts said the fire at Seoul's 600-year-old structure may have something to do with the mysterious "fire-torching" energy from a mountain in southern Seoul.
"From the perspective of Oriental topography, it is possible that the energy torched the fire," said Jeon Hang-soo, head of Korea Oriental Topography Research Center. "Basically, Seoul is more densely filled with the energy than any other cities due to the shape of mountaintops surrounding the city ― spiky and sharp. Mount Gwanak in Southern Seoul notably has the characteristic."
In Oriental topography, spiky mountaintop stands for "fire" and "hot temper."
Noting that Gyeongbok Palace, a home to kings during the Joseon Kingdom (1392~1910), Namdaemun and Mount Gwanak are topologically standing in a straight line. Kang Whan-woong, 74, a professor at Sejong University in Seoul, said "Namdaemun was constructed with the hope of blocking the 'aggressive' and 'fire-inviting' energies from sneaking into the palace."
Ancestors had installed statues of "Haetae," a tiger-shaped legendary creature, at the grand gateway in a bid to suppress the energy, he added.
If the gate had not existed, a blaze would have broke out at the palace and even the presidential office, Chung Wa Dae, the professor said.
He pointed out another two mountains in Seoul with the same characteristic as Mount Gwanak ― Mount. Bukhan in the heart of the city and Mount Dobong in Northeastern Seoul.
Some experts in Oriental topography said the number of crimes in the capital might increase in the aftermath of the gate's collapse.
"As Namdaemun, having served as a guardian restraining the 'hot-temper' and 'easy-fighting' energies from Mount Gwanak disappears, we will see the number of crimes in the capital escalating until its restoration," predicted head of the topography research center. "The restoration of Cheonggye stream has largely contributed to mixing the hostile energy with peaceful one from the manmade waterway, resulting in weakening the violent energy."
Namdaemun was given the status of "National Treasure No. 1" in 1962. The original gate was constructed in 1398, rebuilt in 1447 and has since been frequently renovated.

If that's too much for you to believe, there are other possible explanations. They've arrested a 70 year old man who admitted to carrying out arson. It is said that he is only a suspect, but from what I've read, and from his confessions, I'm pretty sure he did it.

S Korea arrests 70-year-old in landmark fire
(Agencies)Updated: 2008-02-12 10:42
SEOUL, South Korea - Police arrested a 70-year-old man suspected of setting a fire that destroyed the country's top cultural treasure, the 610-year-old Namdaemun gate in Seoul, authorities said Tuesday.
The man, identified only by his family name Chae, was arrested Monday night on Ganghwa Island, west of Seoul, Korean national news organizations said.
"The suspect has admitted he carried out an arson," police official Lee Man-kook said Tuesday, without giving further details.
The fire broke out Sunday night and burned down the wooden structure at the top of the Namdaemun gate, which once formed part of a wall that encircled the South Korean capital.
Police have secured a letter from the suspect, in which he complained about the compensation of his lands in Gyeonggi province near Seoul and he set the fire to draw social interest, Yonhap news agency said.
Hundreds of stunned South Koreans gathered near the charred structure Monday night.
"My heart is burning," Lee Il-soo, a 56-year-old man who runs a small business, said as he fought back tears. He said the fire had destroyed the pride of South Korea.
The two-tiered wooden structure was renovated in the 1960s, when it was declared South Korea's top national treasure. The government built a plaza around the gate, officially known as Sungnyemun, in 2005 and opened it to the public the following year for the first time in nearly a century.
The gate — carrying a plaque reading "The Gate of Exalted Ceremonies" in Chinese characters — had been off-limits to the public since Japanese colonial authorities built an electric tramway nearby in 1907. Japan ruled the Korean peninsula from 1910 to 1945.
The Cultural Heritage Administration said it would take at least three years to fully restore the gate and it would cost some $21 million. Some 360 firefighters fought to bring the blaze under control, said Lee Sang-joon, an official with the National Emergency Management Agency.
Yonhap reported earlier that police said Chae's physical appearance and outfit matched those of a person witnesses said climbed the stairs of the gate shortly before the fire started. It added that police found a backpack and an aluminum ladder at Chae's house that witnesses claimed the man was carrying at the scene. A bottle of thinner was also found in his house, it said.
Yonhap said the man had been charged in 2006 with allegedly setting fire to the Changgyeong Palace in Seoul, which caused $4,230 in property damage. Yonhap quoted the police officer as saying Chae was only one of several suspects.
An official at a police station handling the case refused to confirm the report.
Firefighters found two disposable lighters at the spot where they believed the fire broke out, Yonhap reported earlier, citing fire official Oh Yong-kyu.
President-elect Lee Myung-bak visited the scene Monday and deplored the destruction of the landmark, the namesake of Seoul's central district.
Kim Ok-ja, a 40-year-old public servant, said she could not sleep Sunday night after hearing of the fire because her heart was broken.
"I came here immediately after finishing work because my heart aches so much," she said after offering a white flower, a traditional symbol of grieving.

Also, there is a lot of discussion as to WHY Korea didn't have more surveilance or protection for Namdaemun, their No. 1 National Treasure. 24 hour guards, more cameras or such. Who knows. I think that Korea doesn't have a lot of the vandalism problems that many countries have to deal with.

Poor Security Blamed for Gate Burnout
By Kim RahnStaff Reporter 02-11-2008 18:54
Security loopholes were detected after an overnight destruction of the nation's top treasure Sungnyemun Gate, also known as Namdaemun, in central Seoul.
Experts said that the fire that destroyed the city's landmark wooden architecture was destined to happen due to the opening of the treasure to the public without appropriate security measures. They said the seeds of the accident were sown in 2006 when the gate was open to the people with the exception of second floor of the gate.
Despite the free access to the gate, only six infrared sensors and four CCTVs were installed around it, with no monitoring inside the gate, police said.
Eight fire extinguishers were the only anti-fire equipments for Sungnyemun, according to the Cultural Heritage Administration and Jung-gu ward office. There were neither fire alarms nor sprinklers.
KT Telecop, a security services unit of KT Corp., has been in charge of security of the gate since late January. The security firm failed to spot the fire immediately and its security officials were on the scene later than firefighters.
As to the start of the fire, night lighting equipment had the possibility of short circuiting, while arson was feared as people could easily access the gate.
Three workers from the ward office guarded Namdaemun on weekdays between 10 a.m. and 8 p.m. when the arched gate at the center of the structure was open, and one worker on weekends.
During nighttime, however, the cultural asset was guarded by an unmanned security system of the security firm. Some pointed out that the blaze could have been brought under control at the start of the fire if a night duty worker had been stationed there.
"Three workers are posted there during the daytime, but their main job is opening and closing the gate, providing information to tourists and checking the condition of the structure. We had only an unmanned security system at night, so it was difficult to take immediate measures against fire or other damage occurring at nighttime," a Seoul City official said.
He said a state managed system should have been prepared ― for example, the government directly managing cultural properties or providing an adequate budget to local authorities in charge of management.
After the fire at Naksan Temple in Gangwon Province in April 2005, which destroyed a bronze bell, Treasure No. 479, the administration has promoted a disaster prevention project at major wooden cultural heritages, setting up fire-fighting equipment at four temples so far. Sungnyemun was included in the project, but had not been equipped with such systems.
The official also said that the law on cultural property protection focuses only on "preserving the assets in their original forms," and thus restricts installing fire-fighting equipment.
"To keep the cultural properties' in their original forms, the law allows only simple fire-fighting equipment such as fire extinguishers. Installing equipment using electricity, such as fire alarms, is restricted to prevent possible damage to them," an official of the National Emergency Management Agency said.
Lee Sung-won, deputy cultural property administrator, said Monday that the authorities will make efforts to restore Sungnyemun as it was.
Besides Namdaemun and Naksan Temple, the nation has seen several cases of fire, which resulted in damage to cultural heritage sites. In April 2006, a man set one of the buildings in Changgyeong Palace in central Seoul on fire. In May 2007, fire started by an arsonist demolished a wooden pavilion in Hwaseong Fortress in Suwon, Gyeonggi Province, a World Cultural Heritage. Earlier this year, two middle school girls set fire to a grass field near the fortress while searching for their cell phones.