Can't help but compare this to the uprising in the Middle East over the last months. Sure, the motivation is different, or even incomprehensible in the case of London. Still, they're both begging the question "why now?" and they exhibit characteristics of a highly contagious epidemic, where the original reasons are almost irrelevant. People frustrated for all sorts of different reasons find the opportunity to express violently their anger with much less fear of repercussions than usual, mostly thanks to their sheer number and decentralization. It's fascinating (or scary, depending how close you are).