In France, but also Belgium and the Netherlands, you have a very malcontent population of 2nd or 3rd generation offspring (mostly male) of migrants who feel left out by the system and take any opportunity to cause chaos. It are these kids who set cars alight, not the protestors.

Often when there is a truly large protest, they are there to “fight against the system” by getting into fights with the police and burning cars and just causing overall mayhem.