You see this so often with benign traffic stops that escalate into physical confrontation for no good reason.
Part of the problem is availability cascades.
Think of all the interactions where the cops de-escalate. You don't see those videos.
When he cops back off the criminals feel it and become more aggressive.
Cops being aggressive cuts down and crime but you have more incidents where the cops rough people up.
Under reacting and over reacting both have consequences.
Teaching gives you hundreds of opportunities to over react in response to assholes baiting you.
I'll avoid 99 of them and then when I'm tired, I'll come in a little hot and then I am the fucking asshole.
But I'm like "what about the 99 times, I handled my sh!t in a tough social interaction where the other person was a fucking asshole?"
For a cop this is even more so.
Being in charge of groups of people's safety gives you more empathy for the cops.
Yes, lots of them are assholes