As some here say, it seems that a lot of the trouble makers were there specifically to cause trouble. (Most were just drunken louts; some came dressed in the black uniform of self-styled anarchists.) They are not sports fans!
One thing I noticed, while watching the riots on live TV, was the number of cellphone cameras in play. Most of the crowd weren't actual rioters. They seemed to be hanging around to watch the fireworks — and take pictures. My guess is that there were, at least, 10 cellphone cameras for every criminal thug rioter. A quick search on Youtube produced 1,810 hits for “Vancouver riot 2011. I expect that there are thousands more to come. A lot of the rioters were clearly performing for the cameras.
It's simply the outcome of liberalism.