Opportunity is probably the biggest reason. Many law abiding citizens will turn a dime if you set an opportunity before them and they could get away with it.
The second reason is probably sheer emotion of the moment or heard mentality. Such as when the fans of a sports team burn their own city after winning a title.
You might be right. I feel differently. There is a reason that this hasn't happened in a long time or on this scale. Ironically last time was for the same stated reasons. hmmmm....