I think being constantly barraged with images and stories of police impropriety, corruption, and incompetence does that to folks
Especially when we're supposed to depend on these folks for protection. And the reaction to this incident isn't in a vacuum, people are tired of police abusing citizens and firing off on unarmed people in general and stories like this just accentuate what we're already feeling.
People didn't riot in Ferguson or Baltimore or Baton Rouge just for one incident, it was a pattern of abuse. And we've become accustomed to police violence resulting in civilian death that we react this way now. If you were a regular citizen you'd probably understand more.