Why do protests turn into riots and looting?

Why is it that American race protests and demonstrations (and often those elsewhere too) almost always turn violent and result in looting?
Black protest organizers in Minneapolis, Los Angeles and other American cities have been quick to distance themselves from the looters and property destroyers that have sucked up most of the media attention, and claim that any looting and violence over the last several days is not being done in the name of George Floyd. There has also been a lot of discussion about the difference between damage to buildings and damage to human beings, and about the very definition of the word "violent" (most of the damage to stores and automobiles has come from protesters, but most of the actual violence perpetrated against people has come from the security forces).
It seems like whenever there is a genuine outpouring of grass roots anger, as we have seen many times before, such as during the 1992 Rodney King riots, the Seattle WTO riots of 1999, the 2014 Ferguson, Missouri unrest, all attempts to maintain peaceful, non-violent demonstrations and street protests always fail, and the situation deteriorates into violence against the police (and often back again), the torching of vehicles and innocent local businesses, and the senseless looting of consumer durables stores that find themselves in the wrong place at the wrong time.
Minnesota governor Tim Walz has suggested that up to 80% of the looting and torching in the Minnesota protests in recent days has been carried out by outsiders, although it is not clear how that conclusion was arrived at. Certainly, there were plenty of out-of-state car licence plates in evidence, but it's hard to pin down where the thousands of protesters actually hail from. Other state officials and some black organizers have pinned the blame on white supremacist and anarchist groups who were looking to discredit genuine black protesters or just to destabilize the state in any way they can, but there seemed to be plenty of black looters in action too.
There always seem to be these rent-a-crowd anarchist and neo-fascist types around looking to further their own political agenda at the expense of genuine public outrage and political action. And, of course, Tweets from idiots of Donald Trump threatening protesters with violence obviously don't help. Even relatively thoughtful commentaries from influential media personalities like Trevor Noah, with his "police in American are looting black bodies" video musings, don't really help, quite frankly, because they they can be seen as effectively justifying looting as a tit-for-tat response (although I do recommend you watch that video, it is 18 minutes well spent).
One phrase that has been commonly invoked throughout these street protests (and previous ones) is, "merchandise can be replaced, black lives can not", which certainly serves as an excuse even if it's not a justification. An article entirled "In Defense of Looting", which dates back to the Ferguson riots of 2014, has also been doing the rounds. Among other things, that article claims that, if it were not for the looting, the "white supremacist" mainstream media would not even report this kind of protest; that it is not necessarily morally wrong for poor black people to take nice things if there is no other way they could possibly acquire them; and that it is only stealing from the rich anyway so it doesn't really count. Furthermore, it argues, the largely non-violent civil rights movement of the 1960s only got any real traction for change once things started to turn violent, a contention that I think a a lot of civil rights veterans would strongly disagree with. Anyway, you get an idea of the tone of the piece, although I think it is still worth reading for context if nothing else.
So, yes there will be anarchists and neo-Nazis. Yes, there will be poor people who feel they deserve a new flat-screen TV, or just some food staples. Some of it will be down to the sheer opportunism of poor and downtrodden people who see an opportunity to "stick it to the man", whatever that might mean (one article I read describes riots as the actions of those who have exhausted every other way to be heard", and asks, "what other choice do people have?", although that same article also argues that looting itself is a political act, which I find questionable). Some of it will be down to tempers running a bit too hot in a volatile and emotional situation, righteous indignation gone sour, people doing things they never intended to do. Some of it will just be sheeple following the crowd for no good reason at all. Oh, and coronavirus, just in case you had forgotten about that: lockdowns and layoffs and constant stress make everything worse and everyone's reactions that bit less predictable and more extreme.
As with most things, there is probably no one single explanation for looting and violence at political protests, but a whole host of contributing factors, and probably a smattering of chaos theory.