Hmm... I noticed something.
None of the deaf guys gestured that they were deaf. If someone tries talking to you and you can't hear them, do you start signing? What if that person doesn't know what you're doing and thinks you're getting aggressive towards them? What you would do is point at your ear and shake your head in a "no" motion... possibly along with mouthing that you can't hear. Did any of the deaf guys in the video do that?
Also, the 3rd guy approached one of the officers during the beating. Everyone knows that you shouldn't approach a police officer when he/she is in the middle of an arrest or anything.
Sure, what the police did was pretty harsh and cruel... but who knows, it might have been different if the deaf people actually emphasized that they were deaf.