I waited a few days until if any more video is available. So watching more angles, did he resist while inside his car? Looks like it because a 2nd officer had to help cuff him. But at that point, he was cuffed. He was then walked from his car to the sidewalk and sat leaning against a building while cuffed behind his back. Then he was helped up and taken to the awaiting police car. At this point, he may have resisted further and was taken to the ground.
But I see no reason why a cop should put their knee on the back of someones neck who is handcuffed. Knee on shoulder, back, lower back, would be OK. The neck is a dangerous area. Even if he resist further, it's not like he can get up off the ground while he is cuffed behind the back. At most, he wiggles like a worm (left to right). But on the other side, how many cops are told "it hurts, I can't breath, the cuffs are cutting of circulation" when detaining someone. But regardless, the knee was on the neck.
I think this cop knew what he was doing and thought at most he would pass out and not die. And the other cops watching did nothing because of their unwritten rules. Only on a few occasions do other cops step in if 1 is out of line. Last vid of that that I saw was a guy holding a sign protesting. 1 cop told him to take his sign and leave. Another stepped in and said he has the right to free speech since and can stay.