All I can imagine is long simmering anger, and a thousand little cuts leading to such behavior.
And then there is the group mentality, the safety in numbers, the masks. Protests need not be violent, protests can simply be changing behavior to longer support the violence of other.
I can't tell you how many instances of 'equipment failure' exist in the police forces that wear body cams
The cameras seem to not work in the jail cells either, no matter how high-profile the inmate is.
Instead, we have the President inciting and dividing.
I have several US friends here, but they have already been selected as different, because they are here. I can't generalize the people, but the systems supported are lunacy.