Riots erupt, particularly at European soccer tournaments, on a fairly regular basis. What does social psychology suggest about why this occurs?

Riots erupt, particularly at European soccer tournaments, on a fairly regular basis. What does social psychology suggest about why this occurs? Do you think that American sporting events increase, decrease, or have no effect on subsequent aggression? Among the participants? Among the fans/observers? How have you felt after watching a violent sporting event? In your answer, remark on one of the following:  catharsis effect, the frustration-aggression hypothesis, or social learning theory.