Of course, as has been pointed out already in this thread, there were destructive riots some years ago following protests, but the thing about protestors is that they generally do not, with very few exceptions, consider themselves to be the bad guys or intend to riot. Rioting is what you get when you police a protest wrongly.
Doesn't matter what the protest is about, BLM or Jan 6th or whatever. It's just people being people, and crowd control 101.
Protesters believe they are in the right. They are exercising what they see as their moral right to stand up for what is right and good and is being ignored or trampled down. You may or may not have ever been on a protest but I would invite you seriously to imagine what sort of issue could make you actually do so. You would be doing it because it was the moral thing to do and you would be ashamed not to join.
Policing a protest goes wrong if the police seem to be interfering with the crowd's lawful, moral right to protest peacefully. If the crowd sees what looks like their moral rights being oppressed they are going to be outraged and defiant. Doesn't matter if some stupid jerk on one or other side "started it", the police need to be the disciplined professionals who know about de-escalation and not the bullies who wade in and break heads to stamp their authority on everything. If the police wade in, some people will panic and run away, others will stand and push back. Just people being people.
The unfortunate extra contribution this time is that it's not just the police on the side of the authorities. It's some less professional and less trained and less disciplined folk too. What could possibly go wrong?