The immediate answer is the killing of Mark Duggan, in apparently strange circumstances. A second answer is that there is a criminal element in the Tottenham/London riots.
And it's clear those are part of the answer.
But I'll suggest the relevant question is not that. The relevant question is: are those the only things behind those riots?
"Unemployment jumped by 44,000 in the final three months of 2010 to just under 2.5 million, meaning that 7.9 per cent of workers were out of a job. But the youth unemployment rate hit 20.5 per cent, following a 66,000 increase to 965,000, the highest figures since records began in 1992.(...)
"There were 75,000 people aged 18 to 24 who have not had a job for two years, the figures said. This was an increase of 43 per cent on a year ago.(...)
"Liam Byrne, the shadow Work and Pensions minister, said young people were being hit by a 'triple whammy' as the Government scrapped Education Maintenance Allowances, which allowed some students to stay in education; introduced increased university tuition fees and abolished the Future Jobs Fund - which Labour claim would have created 200,000 jobs." (See here)
I don't know for sure what the answer could be. However, given the figures above, I'll answer as The Guardian's Dave Hill, in his own opinion piece, reproduced locally by the SMH:
London is calling and someone is bound to take the call.
"I don't know what the answers are, but feel grimly confident that such a perfect storm of rumour, resentment and criminality could break in a dozen other parts of inner city London any day. These are nervous times."