It seems perfectly rational, or if not rational at least intuitive, that momentum of a movement would increase participation from people who already agreed with the movement, but weren't doing anything about it.

I guess I'm not totally sure what point you're trying to make, what's your theory exactly?

I'm in the UK. I cannot recall any recent protests against racism or police brutality (apart from the current ones) that I might have joined. There probably have been some, because racism and police brutality are certainly issues in the UK, but they did not get enough coverage to register with me. The most recent protest, prior to the current ones, that I can recall is the Extinction Rebellion. The most recent protest I did attend was against leaving the EU, for which one of my motivations for attending was to take a stand against the xenophobia that I see as a core part of the Brexit platform.