Dog groomer arrested for animal abuse

[Catherine]

A dog groomer on Long Island was caught on camera swinging dogs above his head by their leashes. He dropped them on the floor and even smashed a dog against  fence.

https://www.cbsnews.com/newyork/news/lon...-to-floor/

The groomer has been fired and he has been charged. People are upset because they know their pets were abused.
How was this guy able to treat dogs this way and no one else on staff noticed. There must have been some signs. People who mistreat animals some of the time are unlikely to be good to them the rest of the time. 
Maybe people who work with animals need to be screened better.