It seems that in France, people are increasingly saying that the riots were caused by a combination of race and polygamy, rather more than any directly Islamic rabble rousing. This on the news that a lot of the rioters were Black (and, presumably, often from Muslim, sub-Saharan African families) rather than Arab. I don't know exactly how this finger pointing is playing out in the French media, but the accusation is apparently that Blacks participated out of proportion to their numbers in the French immigrant communities where North Africans are the large majority.
I've argued that we should be careful when attributing causes to irrational violent outbursts, and that our attributions are usually much more about attempting to shape the order that comes after chaotic violence than about explaining its initiation. Resentment and violence are anything but rational, straightforward, "root cause" and effect phenomena. Having said this, I have to add that if the French welfare state has been subsidizing large-scale polygamy in France, it has only been begging for trouble.