Reasonably, they wanted to keep warm. Less reasonably, they did so by tearing down a building and using it to light a giant bonfire in the middle of Main Street. They then proceeded to use the entire town as a makeshift toilet, urinating and vomiting wherever they damn well pleased.
The Summer of Love, now with riot police!
The townspeople and the mayor, who had welcomed the students earlier that very day, politely asked them to leave. The majority politely complied. A significant minority decided to destroy the town instead. The mob first took out their ire on the bars that failed to meet their partying needs. Then, they turned to other business establishments. They left no stone unturned -- or thrown -- and many buildings were hit so bad that they later had to be demolished.
The local police force, presumably consisting entirely of Frances McDormand and armed with polite suggestions, was powerless against the rioters.
"Well, gee, we could really use some gosh darn help here, don'cha know?"