To be fair, they had just come from the front window of a JD Sports!
Class and social distinctions, single mums, violence in music, tv and cinema, boredom, too much cheap alcohol, racial tension, Police being too heavy-handed, Police being too lenient, the weather, David Cameron being a fucking arsehole, having a government that leads by example by engaging in corruption and massive rorting of expenses, parents not being allowed to hit their kids, kids coming from abusive homes, a materialistic society, Blackberrys, Twitter and Facebook have all been fingered at one time or another as being the "cause of all this madness" and I suppose each reason will have contributed in their own way at various levels but to put the blame down on any one thing, it would have to be human nature itself!
Mankind has always loved a bit of chaos and anarchy now and again. You get enough people together in a large group and it's very easy to get them riled up and running amok. Otherwise level-headed people lose their shit an get the red mist ALL THE TIME when they become part of the mob.
If you look through history from the sack of Rome to the French Revolution, the rise of the Nazi Party, Jonestown, Brixton, LA, pretty much every World Economic Forum, Football Hooliganism, Rwanda, Cronulla Beach and any suburban party that's foolishly been advertised on Facebook, people have been gleefully losing their collective shit at the drop of a hat for centuries.
London (and the rest of the country) is just yet another example. It'll might happen here too someday...