Forty years ago Harvard Professor Edward Banfield wrote a chapter in his book about the riots of the 1960's. From what I can tell, almost everything he wrote applies to these current riots in London.
The primary reason that people riot is that rioting is fun. It gives an immense surge of adrenaline and other hormones, it gives the thrill of chaos and action, of the power of mindless destruction. In order for a riot to occur there must be some sort of event that acts as a Schelling point or catalyst. This Schelling point might be a real grievance, or a contrived one. It might be some innocuous event, like a sports victory. News of the riot spreads via the news channels of the day - tv, internet, twitter, etc - and the riot snowballs. The final necessary ingredient is weakness on the part of the police. If the police are outnumbered, and only allowed to manually arrest with handcuffs, then the rioters will go wild, and even more rioters will join as they realize they suffer no consequence for their actions. If the police start beating the hell out of people, or even using live rounds, the riot ends fast.
Banfield's book is online. The section on riots starts on page 211:
http://www.scribd.com/doc/49096486/Edward-C-Banfield-The-Unh...