What variables determine which issues get protested versus which generate riots?

Observing the aftermath of the Ferguson decision I have been struck by a couple of thoughts or observations. For all the outrage being reflected in selective locations and in the press, the overwhelming majority of the population appears to be confident that justice was served with most of the surveys I have seen indicating that 85% of the survey participants felt like the Grand Jury had reached the correct decision as opposed to 15% thinking otherwise.

The first observation is the seeming similarity between these protests/riots and similar cycles in the past. In the late 1990s or early 2000s it was riots related to global trade and globalization in general: in Seattle, in London, in Vancouver, in Italy, in France, in Greece - they were everywhere for two or three or four years. Rioting and looting and pictures of neighborhoods trashed and in flame. And then they were gone with not much trace.

Similarly in the later Bush years with war protests, though I don't recall any of them evolving into riots.

Then there was the ripple of sit-ins, live-ins, etc. accompanying Occupy Wall Street for a couple of years around 2011. In the news a lot and then gone with barely a trace again.

And what about the dog that didn't bark? No riots that I recall when the Zimmerman trial concluded with a not guilty verdict. Much the same level of outrage in the press regarding dissatisfaction with the outcome of the process even though the overwhelming majority of the population felt that justice was indeed served. Why no similar wave of protests and riots?

So what gets protested and what determines when protests evolve into pointless rioting.

Perhaps it has to do with the second observation. In the photos of the Ferguson protesters and rioters, take a look at the signs. Only a small portion appear to be focused on Michael Brown and the decision. Many, if not most in some photos I have seen, are actually signs related to all sorts of marginalized protesters: Radical Environmentalists, various factions of Communist/Marxist/Maoist protesters, Community organizers such as the former Acorn, Radical feminist groups, etc. It is almost as if the protest is the catalyst for those who want to protest, not for who cares about the particular issue.

And then there are those who are simply taking advantage of the event for their personal ends or entertainment. I don't see those so much in the photographs but you hear it on the radio when they are doing their man-on-the-street interviews. People who don't care about the issue, they care about having some fur or looting.

I have no idea what the breakdown might be but it does seem like, just as with the globalization protests, the anti-war protests, the OWS protests, that, to paraphrase Julius Caesar, all protests are divided into three parts: the looters, the ideologues, and the genuine protesters. And as with globalization, etc. it feels like the first two parts are the majority. Ideological and anarchical opportunists.

So I guess I have three questions.

What determines what gets protested? Why Fergusson and why not Zimmerman?

Why is there such a disconnect between the citizens (85%) and the clerissy and protesters (15%)

Why do some protests attract the anarchical rabble while others stay focused on the issue (whatever that issue might be)?