I don't like the framing of the behavior as somehow irrational or random or clinically-diagnosible. It's not irrational to get involved more in a movement when it
is a movement, and seems like it might be gaining traction. Just like it's not irrational to not-boycott some useful, cheap product even if you strongly disagree with some of the practices that go into its production, but to join a boycott if it starts and seems to be gaining traction. In fact, it's less rational and more tied up with self-image than with utility to do the former—and
even so it doesn't mean that people who do things like solo-boycotts are wrong, or bad, or misguided, at all.
[EDIT] to put it another way, there's a cost to political action. That a lot of people might see that cost WRT anti-police-brutality protests as lower, and the benefit it yields higher, this week than they did two weeks ago, and so be willing to participate, is not "emotional hysteria". Not that nothing that could be described as that has been involved at any point in the process, but that seems reductionist to the point of being very wrong as a characterization for the overall movement.