Vaughan Bell explains it using the Elaborated Social Identity Model of crowd behaviour, basically a fancy academic phrase meaning "them vs us":
The problem police face is that in most large threatening crowds only a minority of people are engaging in anti-social acts. Lots of people ‘go along for the ride’ but aren’t the hardcore that kick-off without provocation. If the police wade in with batons indiscriminately, lots of these riot wannabes suddenly start to feel like they’re part of the bigger group and feel justified in ripping the place apart, mostly to throw at the coppers. Suddenly, it’s ‘them’ against ‘us’ and a small policing problem just got much much bigger – like attacking a beehive because you just got stung.
(Photoshop from here.)
The Dish 