The rioting is caused by a fairly small group of assholes -- basically a rent-a-mob. They go to these things with the specific planned intention of rioting afterwards. And once it starts...
I'm a little amazed that the cops weren't more prepared though. The "riots after the game" has been a regular topic of conversation for the last couple of days. Can't believe anyone would have been taken by surprise by them. Also wonder why the stores didn't do more to prep, like boarding up their windows or taking the good shit out of the windows. This is not rocket science.