In the majority of protests around the world, there is always an element there to cause trouble. They have nothing to do with the main protest at all. Some are even professional protesters.
Take the G8 summits, for example, or even football supporters. Very small minorities of supporters are not supporters at all but will go to the away matches just for the fights they like to cause.
Take the protest of this past weekend. Were the trouble-makers truly protesters or only there to cause trouble? I believe the latter.