PS The fat guy does act as a catalyst, he's pretty funny cos he's fat but he's uninhibited. But the SOURCE, is the gay guy dancing. There's a key difference between his earlier dancing and the dance that causes the riot. Its a big indicator of open confidence, a power display. Arms in the air. Fully extended, for a long period. See how everyone becomes uninhibited and adopts this normally socially difficult dance. Usually only really happy self focused people who are free from feelings of social judgement will consistently dare to raise their arms up. It is a big attention draw. Also the jumping and 'Whooping' is another unstifling action that breaks normal social levels of individual involvement and frees the people to act uninhibited.