From the Los Angeles Times:
Though demonstrations over the Zimmerman verdict have been mostly peaceful across the U.S., California has stood apart, marked by violence in Los Angeles and Oakland, where officials' and activists' patience were diminishing.
The conventional assumption about riots appears to be that they occur in proportion to the aggrievement of the "community." In contrast, my impression over the last four decades is that they more occur due to "Hey, the cops aren't a worry, let's get free stuff!"
Thus, I would distinguish between protests and riots.
And I try not to make predictions about when riots happen because they tend to be dependent upon two things happening. A lot of people in the street and the police losing control. The latter tends to be hard to predict.
A key example would be the huge 1992 Los Angeles riot. Was it caused more by the passionate outrage of the community ... or by the LAPD sulking over all the criticism they'd undergone, and thus wimping out when shoplifting at liquor stores at the corner of Florence and Normandie started to explode? I'd lean toward the latter (although I can appreciate arguments for the former).
Since then, riots in North America seem more likely to be associated with sports than with traditional complaints. The big riot in Chicago in June 1992, for example, came as a surprise surprise because the crowds weren't angry, they were ecstatic over Michael Jordan's Bulls winning their second straight NBA title. There hadn't been a riot the year before when the Bulls finally made it to the top, so why the second year? And then, in 1993, there was some looting but the Chicago PD was well prepared with cops on horses.
Thus, if riots are largely the result of cops failing to enforce order, then the increasing professionalization (indeed, militarization, as seen in the Hunt for the Bomb Brothers) of police forces over the last generation, especially since 9/11, has made riots less common.