Social historians, looking at the many protests against police violence (protests taking place across America and in other countries) will be able (if imprecisely) to assess the motivations of these recent events.
Of two things one can be confident, without being a historian: these protests are in response to specific acts of law-enforcement violence, with evident racial bias, against specific people, notably the late George Floyd, but many others, before his death. (Twelve-year-old Tamir Rice, for example, was killed six years ago, in 2014.)
If no one had been injured or killed, especially having been injured or killed in evident and disproportionate numbers through racial bias, there would have been no protests. This is simply to say that the protests are grounded in real acts of violence against real people – these protests are a natural and legitimate response to injustice.
There’s one other point on which one can be confident: protests against these acts of specific, racially-disproportionate government violence are taking place after three years of Trumpism.
The simplest answer to why now is that civilized people saw violent, disproportionate police action against George Floyd, among others, and could endure these brutal offenses no longer.
It’s reasonable, however, to conclude that these offenses have been made worse for many people through the legitimate concern that Trumpism would excuse – and even approve – more of the same forever.
Generations have been too long; forever would be unbearably worse.