This, like so many of these stories where cops have begun to be less like cops and more like an out of control gang, all have a common foundation. I think you can trace it all back to when police departments began to unionize. Pre-union, a cop has allegiance to the public. Post-union, a cop is no longer responsive to the public, his allegiance is to his union. Worse, the union, by design, does not deal in individuals, it deals in the the group, the mass, the mob. Individuals are no longer responsible.
Psychologically, over time, this has to have a corrosive effect on the police force. Instead of responsible individuals you now have group-think union thugs who know they can push the city and the citizens around. This is a bad development. Stories like this one, and others, are the outcome.
What year did unionization begin, on a large scale?