Let me throw out another hypothesis. I will call this the "roller coaster" hypothesis. People in the US have pretty much everything they need and most of what they want. If they don't they have no one to blame but themselves. They are secure. They have no realistic threats.

This condition is antithetical to the human condition. We have developed to be able to deal with challenges. We have "fight or flight" capabilities built into us. Now, for a long time we have found outlets for this need for the fear of a near death experience. Scary movies and roller coasters. "The Walking Dead" is very popular. In the end, it is a story of survival at a very animal level. But eventually all that fails to satisfy. So we begin to create the chaos that we need to feel alive. We create a conflict between the police and the citizenry that really has no substance other than the feeling that we are doing something that is life-or-death and is important. It is the civilian equivalent to what a Soldier feels in combat, something that has been idealized in the public eye in the last twelve years, but only a very limited portion of the population has really felt.

OK, I may be nuts, but I thought I would throw it out there.