Why do people riot
There are several reasons that result to people rioting. However, the major reason for riots is dissatisfaction from decisions made. Peer pressure from other rioting people can make some guys to also result to rioting.
They riot to tell the government they are bad at their job.
People riot when their favorite team wins the championship. They riot when their favorite team loses the championship. Mostly it's an excuse to act like savages and break stuff.
To protest, or perhaps because a large group of people becomes overly excited for something like sports matches or concerts.