The general consensus from Critical Mass rider you were behind them (yes, they are biased but I've talked to them) is that what you see is what it was: Riders were going around the officers, the one officer started to move forward, the rider moved a bit to the left to go around the officer (but wasn't swinging EXTRA wide) and was then, basically, jumped.
Up to that point, nothing was happening that has not happened at all of the critical masses before. Nothing was different.
I am NOT saying this is a case of NYPD striking out at Critical Mass. Yes, they are hyper-sensitive and will arrest for the slightest provocation beyond bike riding. But this appears to be a case of an officer who had only been on the force for 3 weeks, is only 22 years old, and got over excited – to the point of randomly selecting a rider and pushing them.
I obviously have no idea of all the details. I do know from my previous experiences with the NYPD at similar events (Idiotarod, etc.) that 95% of police officers are supportive of such events... as long as nothing unexpected happens. But there are also always the officers who seem to approach it, visibly pumped and agitated.