No, we don't know what was said - but I could see the black cop motion more than once for him to stand up and comply. We also don't know what was in the bag... but no matter what, resisting and refusing to cooperate, even if the cops are in the wrong, is going to result in bad things happening. It's always in your best interest to comply, even if you think they are stupid.
Percentage-wise, most cops are good people, but there's always a few bad eggs, and some are jaded and just assume everyone is up to no-good, although I do find it interesting that the black cop is the one who used enough force that the white cop was trying to stop him by grabbing his arm once the guy was on the ground. Apparently he thought the other cop was being too rough, too (I re-watched it.) With out hearing audio, and seeing it from another POV, it would be hard to say if it was excessive force, or not.