Maybe she just wanted to speed up the base relocation?:

A South Korean woman was arrested Thursday on suspicion of setting fire to a wooden training facility at the U.S. military headquarters in Seoul, which injured three people, police said.

The fire, which broke out at 1:39 a.m. Thursday at Yongsan Garrison, quickly spread and burned down four other U.S. command buildings, before it was contained three hours later, police officials said.

Now why was she on post at 1:39AM to begin with?:

Police apprehended a 57-year-old South Korean woman who was wandering around the area after she admitted to initiating the arson attack in protest of “U.S. terrorism.”
“I sneaked onto the U.S. base and started the fire with a cigarette lighter to punish (U.S.) terrorism,” police officials quoted the woman, only identified by the initial “K” of her surname, as saying.

If an old mentally deranged women can sneak on to Yongsan imagine who else is on post.