Monkey see, monkey do...oh, and alcohol is usually involved heavily...
ETA - Listened to the sports-talk guys chat about this...the interesting thing being both the lefty and the righty agreed it was the media that stoked up the rioting meme...one picked on the local Ch.9 clown covering the stuff live, said he was practically begging for students to push him and get some mahem caught on camera, seemed manic that something did happen...that all but one of the local stations made a bigger deal about this and basically turned a handful of drunken idiots into a 300-400 person rampage...if the media had just stayed the hell away and not said boo...well, they can't do that, now can they?!
One guy speculated that the idiot that had the street sign tossed through his windshield either a) left it there hoping something would happen to it or b) was the one who did it, all for the insurance write off!