>Riots, at least those in recent events are an example of contagious social behavior. That is evidenced by the fact that the blamed issues have been known problems for longer than two weeks with people actively working for years to raise awareness of them.

I also believe rioting is contagious. But that doesn't mean it doesn't require justification. Otherwise, wouldn't we see riots totally disconnected from any justification? But few people, when they see vandalism and don't agree with the cause, won't join in. Not enough to spark a riot.

>Why suddenly congregate into emotional hysteria, especially since other people have been actively addressing these concerns for years, when the problems of complaint are preexisting and were clearly previously known to these congregants?

There have been protests, lawsuits, smaller violent reactions, and other riots for decades. It doesn't seem sudden to me.