And the cops helped start the riots by throwing rocks at protestors. http://www.huffingtonpost.com/2015/04/27/police-throw-rocks-...
From the video on the link you referenced, it looked like the cops were in full retreat from a wild crowd throwing objects. One of the cops half-heartedly tossed something back... inappropriate response on his part, but hardly evidence that he was the instigator in anything.
Protesters do a bad thing: "Pathetic rioters! Ungrateful youths! Sit down and do as you're told!"
The poster to whom I was replying stated that the cops "helped start the riots" by throwing objects. I pointed out the problem with that argument, but I'm not excusing the police categorically.