I don't think there's a single cause. In Tottenham and Croyden it can be traced back to the shooting -- the local police having garnered a slight reputation, no doubt solely from a few bad apples, of being unduly trigger-happy and/or a bit racist. (Jean-Charles de Menezes, Stephen Lawrence.) If you think you're being treated as an second-class citizen, that's a fair enough reason to be a protester, and protests over less have turned to riots in the past. (Though that was before the IPCC was invented; in this case the IPCC have already refuted the original story that Mark Duggan had shot a policeman before being killed.)

Everyone else's rioting, though -- Manchester, Birmingham, Liverpool, Leeds -- is fairly clearly a response not to political or civil-rights issues, but to media coverage of looters gleefully making off with large televisions and not (immediately) getting arrested for doing so. That's straight-up-and-down crime, and is also tragic and sad but in a different way. And in fact you have to wonder whether it's being over-reported: to what degree is a total of two burned-out cars and a slightly pillaged footwear shop, in excess of the average daily amount of crime in a city the size of Manchester? Is it only news because it seems to make some sort of pattern with everyone else's crime? It'd be wrong to not report the rioting, but the very existence of the reporting, however dispassionate, itself can feed back into further unrest.

Incidentally, there are a lot of police mooching round Cambridge this evening.

Peter