You have a large population of soldiers, many who come back home after their tours of duty, where they were in effect an occupying force. And then they get a job in law enforcement. They are used too a certain way of doing things in Iraq and Afghanistan, and they bring that culture home with them. Even for those who didn't serve, as the article notes, this "warrior culture" has bled into police forces everywhere.
In the military, they say "train as you fight." Well guess what they've been training to do? Hint: it's not the warm and cuddly community policing. When you're used to "fight or fight," in the military, and then you're put back in the civilian world, these issues come to the fore.
Not to be mean or unfair, but some of these soldiers who come back have PTSD and other mental issues (not their fault), but I've wondered how much of that also plays a role in police violence and the way police officers handle themselves in stressful situations.