According to the Police Executive Research Forum (PERF)
The Police Executive Research Forum, a research and policy group whose members include commanders from the largest U.S. police departments, said officers generally receive far too little training in de-escalating conflict and often are embedded in a culture that encourages them to rapidly resort to physical force.
I interpret this to say that many cops are quick to ratchet up a conflict instead of cooling it down.
UC Berkeley did a study of police confrontations with Occupy protesters and again found that police can control the "tempo" and outcomes of the encounters.
Police Often Provoke Protest Violence
The Bland case is another in which the COP escalated the situation for no apparent reason.
Video of Sandra Bland Arrest is a Lesson in Police Escalation | Houston Press
Roid Raging Police? Maybe this is the reason...
Are steriods behind worst police abuses?
Whatever the reason, video monitoring seems to be only a good first step in controlling OOC Cops.