What makes people riot? The mob mentality?

Because ultimately people in that kind of situation don't think. These type of people are just toddlers in grown up bodies. They were never taught that the world doesn't revolve around them. It is like a four year old throwing a tantrum on the floor because he wants candy except on a larger scale. Unless people like this have outside restraint put on their actions they will not restrain themselves. Look at the actions of Hamas and others in Gaza. They refused to restrain themselves and now are angry that Israel has decided to provide that restraint.

It's referred to as deindividuation. People focus less on themselves and feel like they are less responsible for their actions. It can lead to antisocial behaviors, but usually in the prescence of antisocial cues, prosocial cues may actually increase prosocial actions in these cases. I can't see myself ever doing that, but honestly, most people can't. This is because as humans, we underestimate how much our behavior is really influenced by circumstances rather than our beliefs.

most people riot when stuff doesnt go they way the wanted. or if ther is no cops. no cops =norules to the human race. heres an example: if obama would have lost then many people in many cities would have rioted. it wouldnt have proven anything though

A diffusion of responsibility.

When you were younger did you ever pick on another kid as part of a larger group, and whilst doing so think nothing of its moral character? Then later, realize what a dick thing that was to do?

Same principle, extrapolated to infinity.