increase in police brutality

I saw something on the tv where some cops were beating up a suspect inside a jail. It started with Rodney King and in recent years the number of incidents has risen sharply. Ferguson, New York, Baton Rouge, Sacramento which I guess is why the Black Lives Matter Movement started. Begs the question: Why the sudden increase in police brutality cases? Has our society become more violent? Are police not receiving adequate training to deal with the changes in our society?