It is hard to say what was going through the cops mind, and yes he did have a split second to react.

However, if the motorcyclist had a gun and was pulling it on the cop, he would have done so on the left side of his body (think pointing the pistol below the left armpit).

To turn right all the way around on a motorcycle like that is VERY difficult (I know, I ride, well rode before I sold my bike recently). If he were left handed, then his right arm wouldn't be jerked back like that and have been a perceived threat of holding a firearm.

Now, if you have ever been on a motorcycle and pulled over. You know that with your helmet on, you have limited range of vision and almost no periphery. It's possible he heard something (a noise) come from behind him on the right and that caused him to turn to see what it was. I know that lately, when I've seen people be pulled over, two cops always approach the vehicle.

Lastly, when I was pulled over on my motorcycle once late at night around 3am I was scared shitless. I kept my hands on the bars, waited until the cop got next to me and then asked if I could lift my visor. Then, after he could see my eyes (and the fear in them, lol) I asked if I could remove my helmet. Cops do presume a lot with motorcyclists, as a helmet can be used as a weapon (it's heavy and the straps can act as a swinging device). But that's also because of the preconceived notions of motorcyclists being law breakers with speed and driving wrecklessly, which sucks for us.

But without knowing EXACTLY what was going on in the video, it's hard to say anything. I'm only commenting on how I perceive it to have happened. It is definitely ashame that it went down that way, hopefully the rider gets a large settlement and doesn't have to worry about not working the rest of his life.