As per usual, most of the protest tends to always be peaceful. That was true even Ukraine or the arab spring, for example. However, when there are many angry people on street and police/military get involved, clashes inevitably happen.So, um, wtf is going on in california? I've read some articles on this but what I've read varies between protestors either being a murderous zombie horde or harmless hippies with flowers.
In this case, protestors have a very legitimate reason to be angry and antagonistic towards law enforcement, because ICE keeps raiding the neighbourhoods, just ransacking people when it suits them. These legitimate protests have now been met with violent opposition by trump sending the troops and rightwing media vilifying them as left extremist or gang insurrectionists.
There definitely is some violence from the protestor's side, both against law enforcement and property, but this whole operation can only be understood as Trump seeking a confrontation because he wants to call this an insurrection. That is why he deliberately stokes the flames. Burning police cars are a consequence of that.