My personal theory is that the riots are the consequence of the combination of welfare state and absence of cultural integration. The riots mostly happen in places which have immigrants and recent descendants of immigrants living in compact communities and usually those are not the "best" neighborhoods. Welfare state support allows them to live in these closed communities with relatively little integration with outside economy and thus without real integration in the wider society, while absence of the interest of the rest of the society in cultural integration - sometimes represented as multiculturalism and respect for different cultures, but it can be taken other way, e.g. "we don't care what you do there where you live, provided you don't go where we live" - makes these communities alienated from the wider society. This creates an impression that a) the source of the welfare is the society and the state (welfare, government-sponsored enterprises and programs, etc.) and b) the society, which is the source of all wealth, does not want those people to participate is fair distribution of such wealth. In other words, they are in the position of a poor relative, that would not be allowed to die of hunger, but also is not respected and is not regarded as somebody worthy of respect or consideration, and all his worries can be alleviated by giving him a little money.
Situation becomes even worse when second and third generation is raised in such conditions - they do not have any alternative cultural links to "old country" or community brought from pre-immigration times, and all they know from their birth is being treated as outsiders which need to be fed and need to sit quietly aside. Of course, while first-generation immigrants may be content with this, provided they got much better conditions than they had in the original country, the next generations have no such base for comparison, so for them current situation is not an improvement on anything, it's just bleak. Since they feel disconnected from the rest of the society, democratic ways of expressing their discontent appear either unavailable or inefficient. Thus their frustration spills into violence.
The situation may become even worse when the economy is on the downturn and the welfare support has to be decreased. Since many people rely on it for subsistence - as either main or supplemental income - their whole life, reducing it feels like further insult from the society - not only they do not want us to be part of it, they don't even allow us to subsist properly!
Given this theory, countries with bigger, more concentrated and less integrated immigrant populations would experience such events more frequently than countries with more integrative immigration policies and less widely-available welfare programs. I think it is possible to say that in the USA both factors are true compared to Europe - in the US it feels there always were more push for the immigrants to integrate, and the welfare system is relatively less generous and widespread, which also serves as a push to integrate more into the economy and thus in the general society.
Though of course not only immigrants would experience such problems - for example, in the USA some part of the African-American population seems to be in the same condition. The ill-fated "projects" social experiment seems to create similar challenges for its inhabitants, which leads to generally low quality of life, high crime, and yes - violence.
Of course, this is just a theory, since I am not a sociologist and I did not collect hard data to prove it, so I base it mostly on anecdotal evidence, press reports, some books and articles I've read, and other various "soft" evidence. So you are welcome to criticize it.