I didn't see in the video where they ever had him in a police car. If they had him in the one behind the SUV, then they perhaps moved him so that they could leave the police SUV behind the soon-to-be impounded suspect vehicle. Not the best idea given what happened but that's the only thing that makes sense to me.

Edit: Yeah, I went back and rewatched the most complete video I could find. It looks like they had him sitting on the sidewalk relatively relaxed and then he flipped out when they took him to the police car parked half a block behind and to the left. When trying to get him in while he was freaking out, they tried to allow him to get in on his own and he refused. The officer wearing the camera went to the other side of the SUV and opened the door, leaning in to pull him through. Floyd then struggled/kicked himself through the SUV and almost knocked over the officer trying to pull him through. He then died in custody on the ground.

I didn't see in the video where they ever had him in a police car. If they had him in the one behind the SUV, then they perhaps moved him so that they could leave the police SUV behind the soon-to-be impounded suspect vehicle. Not the best idea given what happened but that's the only thing that makes sense to me.