The problem is with how these incidents are dealt with when they happen. Ordinary citizens could expect jail time for doing something like this, and that threat helps to protect black people against these bad people.

In this specific case, it looks like the police officer will face justice. But that only happened after the huge public outcry, and it didn't happen in countless other cases even when there was a big-but-not-as-big outcry. In many cases, not only have the officers escaped trial, they've actually kept their jobs!

That's what people are protesting.

The phrase “the straw that broke the camel’s back” comes to mind.

Yes, outrageous police conduct has happened before. But for some ineffable reason, this one time was one time to many.

I think that because this incident was so clearly visible, the video capturing what was so obviously murder, that it was a concrete platform to start a movement on. In many other cases, there was at least an attempt by the victim to defend themselves, or parts of the video that left holes in the context of what was going on. This one was a slaughter, and no one could find a justification for it.

Salience and specific evidence is a necessary but insufficient condition for these kinds mass outrage conditions. In the end the final trigger is obvious only in retrospect.

As a less politically sensitive topic for most Americans, I point to the Arab spring uprising as a good example. Tensions had been rising for decades, but the final trigger was when Mohamed Bouazizi, a street vendor, lit himself on fire to protest the local police stealing his possessions & threatening his livelihood. A tragic event, for sure, but after decades of brutality and continual tragedy, who could have possibly predicted that that would be the final straw?

Yeah, I think this is it too. In particular, this case was so clear cut that it enraged white people who were perhaps more passively supportive in other cases.