I read an account from one of the neighbors that gives a little more back story. Take it with a grain of salt because it's off the internet and unverified.
This was a private community pool in an upscale neighborhood. It started out as a small party but due to social media, more and more people arrived who were not residents, not friends, and not invited. As more people arrived, they ran off the neighbors trying to use the pool. Some who couldn't get in, started jumping the fence. Some were drinking and smoking pot. At some point a rapper showed up, hooked up his loudspeakers and was blaring profane music.
The community security guards and some of the neighbors tried to put a stop to it and they were run off too. Then some of the kids started throwing bottles at passing cars and damaging property.
The cops then showed up and that's part of what is shown on the video.
I think the one cops treatment of the girl was a little excessive. Police are trained to get control of the situation first, and he looked like a guy suffering sensory overload with so many people running around..
I get that the police protect and serve all of us but that includes the neighbors too. If I had a bunch of hooligans show up next to my house blasting music and throwing bottles at cars and people, I'd yell at them to get back to their section 8 housing too. It doesn't excuse unprofessional behavior by the police, but the idiocy and lack of respect for authority that incites all of this to begin with never seems to get reported on.