Police attacked this person because:

[Riot]

This article says nearly 20 were arrested, and the response from the NY Times over the police treatment of their credentialed reporter:

Video: NYPD Blocks NY Times Photographer From OWS Arrests
http://gothamist.com/2011/12/12/vide...ialed_phot.php This includes another view of the arrest, above, and the police coming after the NYTimes Photog, too (his NYPD issued press pass clearly visible and worn as it should be)

[UPDATE] Andy Newman, City Room's bureau chief, has confirmed that the photographer is in fact freelancer Robert Stolarik and that he was working for the Times this morning. We asked the Times if they plan on releasing a statement regarding the incident in light of a letter the paper signed, along with many other New York media outlets, condemning the NYPD's treatment of the press during Occupy Wall Street.

NYT says: "The directive Commissioner Kelly issued reiterated that the police are not supposed to be interfering with the media’s ability to do their job and cover newsworthy events. We are disappointed that it seems, in this instance, not to have been followed or implemented on the ground."

This is the story the reporter filed on the story. He describes excessive force and 10 men 7 women arrested:

http://cityroom.blogs.nytimes.com/20...etro&seid=auto