Dancing is basically the most dangerous and assaultive thing one can do in public. Especially if one's doing it while black. Just watch this video for evidence.
Alexander Bok, who's a performance artist/prankster (I don't know if that's a real job or just a YouTube title) was trying to take part in Ellen's "Dance Dare" when he chose to do a little dancing behind some police officers on the streets of New York. It's a good thing he wasn't carrying anything that looked even remotely like a weapon (such as a pack of cigarettes) because the response to his dancing is so completely disproportionate to what he was doing that it's mind-boggling. Except, of course, it isn't.
It's clear that Bok isn't dangerous a couple of seconds in when he tries to explain what he's doing, but suddenly there are six police officers on him and they're yelling and calling him an asshole and pushing him to the ground. Bok is a prankster, but if this is how the police react to him just for dancing, imagine how they might deal with someone who was mentally ill and/or less able than Bok to explain themselves well.
I recognize that the NYPD is on edge right now because of recent events, but reactionary assault is simply not any way to prevent crime. And again, there is no crime here.