Why do people riot during peaceful protests or happy gatherings?

I've noticed that a lot of times, when there's a large group of people, riots usually ensue. Even if it's born from protests AGAINST violence or from happy times like your home team winning the world series/cup/superbowl. These are especially common in football(eu[no offense, but it's true]) and I seriously question why people have the urge to hurt others, destroy property, and loot stores for something that's GOOD.

Chameleon2016-11-09T09:17:42Z

Attention

sha wei sted2016-11-08T22:23:53Z

They are not part of the peaceful protest, and the gathering is an excuse. They simply don't care or have no self control.