Blaming it on welfare is just ridiculous.
First of all, I'd separate riots from looting.
I think riots are usually started by groups of people who feel discriminated/underprivileged, and they usually start when something potentially scandalous happens to someone who these people consider "one of us", as they consider it a proof of discrimination/being treated differently.
I think looting on the other hand is a different story. People will loot simply because they feel the risk is low. Looting starts as soon as people get the impression that police etc. do not have control over the situation any more. That's the case during riots, but it can happen in completely different situations as well (think of Iraq when the US troops came for example - as soon as people realized that the police isn't working anymore and that the US troops won't interfere, pretty much everyone started looting)
If looting happens during riots, the rioters will obviously be the first to loot, but as soon as other people see that police has lost control and the risk of being caught for looting is low, lots of people will do it even if they would have never taken part in the riots.