What's happening? What causes all of this?
The core of it is that police are expected to clean up society's ills.
I'm a solid middle class white male. My demographic crime rate and life expectancy is identical or greater than that of western Europe. Even with my access to guns, increased risk for alcoholism, etc. I am likely safe and gonna live a long healthy life.
Poor american, which is largely black, doesn't have the same.
Poverty begets poverty. And with it comes obesity, mental health problems, crime, etc.
So we start with expecting to criminalize everything and send police out to jail everything in response. That sucks for everyone. That can't be fixed with training. The training for what? On how to better deal with being perpetually short staffed and endangered due to societies foundational problems?
We have then magnified that problem. We have instituted mandatory sentencing, militarized the police forces, used federal terrorism laws for domestic drug issues (80,000 patriot act taps via Sprint one year for drugs...none for terrorism), etc. We have created a "us vs the populace" mindset for police. That officer is stuck thinking, "If I don't use 'command presence' I don't get to go home to my family tonight".
Police departments definitely have their own issues regarding militarization, etc. But they are mostly just emblematic of using the wrong tool as a solution to the problem of the symptoms of poverty.