Without a long history of white police treating black people badly, a single incident would not have sparked such protests.
Without a wildly higher-than-average rate of violent crime among young black men in some areas, cops in those areas wouldn't be having to face every situation like another one in which they might get killed doing something simple like a traffic stop. And do you really think that there'd have been riots in Ferguson if the breathless media reports immediately in the wake of what happened had been reporting the observations of credible witnesses instead of the absurdly transparent lies of the criminal running buddy of the guy who had just assaulted the cop? But why did the crowds there, and social media, and some mainstream media outlets catch on fire with the obviously false narrative? Because the honest people who saw what happened were afraid of what would happen to them if they got caught telling the truth. Those witnesses weren't afraid of the cops (they went to the police as soon as they could do so quietly), they were afraid of people in their own neighborhoods.
What sparked violent protests was a bunch of deliberate BS that got trotted out in an attempt to gloss over what that 6'-4", 290 lb "sweet child" and his store robbing, warrant-out-for-him sidekick had just done. If he hadn't stood there in front of cameras and spouted a bunch of self-contradictory nonsense about Wilson shooting out the window of his cruiser, or chasing Brown down and shooting him in the back, or shooting while he was on his knees with his hands in the air and on and on about stuff that did not happen, don't you think that might have been a little different? If the people who live right there weren't so scared of guys just like him and Brown, don't you think the many witnesses who were standing right there and saw what actually happened might also have been on video, talking down the idiots? That would have been great. But they're scared - for their lives - of the very people that the cops also have to confront on a regular basis.
You're right, it's not a single incident. It's years and years of people growing absolutely terrified of the rudderless, violent young men in their own neighborhoods. And when the cameras role, those voices of reason are nowhere to be seen, because they don't want to be another statistic in the huge problem of black-on-black violence - numbers that completely dwarf even the most demonstrably real cases of some dumb cop (white or black