I gotta say, I lurvs me a good cop bashing, but if the cop in question hadn't seen the guy go down, by the time he flipped the grenade in there all *I* could see on the video was a large yelling crowd. Th folks rendering aid did so with such gusto as to hide the folks kneeling and the injured guy.
Situational awareness and peripheral vision is often severely hindered by pro masks. I could actually buy that he was watching the other way (where more pyro was going off) and looked over in time to see a large crowd yelling when he had been ordered to disperse crowds so he tossed some CS over.
Must be my inner CSD coming out.
Also, as other's have said, those are NOT flash bangs.
ETA: $10 says the initial canister skipped. I've skipped full on grenades by firing them at too low of an angle, the weapon launched canisters are pretty easy to have that happen with. remember- Less Lethal, not Non Lethal.