Police attacked this person because:

[Riot]

Interview with sister of guy in the above video

http://www.salon.com/2011/12/13/busted_for_tweeting/

Busted for Tweeting
Police escalation in New York as my brother and 17 other people are arrested for observing an occupation

That was when everything escalated completely out of control. The escalator was stopped. Suddenly, the outer circle of cops was swarming in and violently pushing people away. John had been standing near the crowd, taking video. I was about twenty feet from him, and when I looked back in his direction, I saw his blue hood on the ground. I ran towards him and slid to the ground, leaning in between people’s knees to take pictures. John was face down on the ground being handcuffed, his glasses flung across the floor and people screaming, “Stop, stop, he didn’t do anything!”

A cop pulled me up by my shoulders and told me to step back. I said, “He’s my brother.” Several cops pushed me away as I asked, “What is he being arrested for? He was taking pictures.” A cop said, “He didn’t produce an official press pass, so that means he was resisting arrest.” I quite literally didn’t understand, so I said, “What?” At that point, the same cop said, “If you don’t step back immediately, you will be arrested too.”

I was pushed behind a line of police. I stood there, several inches from them, and heard myself saying, “Why are you doing this?”

A protester next to me was yelling at the cops, something about free speech or unnecessary force or any number of logical things to say at a time like this, I was too distracted to pay attention. But then, an officer said to him, “Get out of my face. I have a gun, and I don’t need people up in my face like this.”