He had a lethal amount of fentanyl in his system, a heart disease, and coronavirus. Chauvin was kneeling on the back of his neck, but the first autopsy found no evidence of asphyxiation nor physical damage. If Chauvin asphyxiated Floyd, it seems highly unlikely, perhaps impossible, that he would have done so without leaving any signs of damage. Moreover, Chauvin had two knees on the ground at once, so for all we know his knee that wasn't on Floyd's neck was exerting most of the force.

I do think it's a tragedy Floyd died, but I don't think Chauvin murdered or even indirectly killed him. I can't see Chauvin being convicted for the former, and probably not the latter either.

There were later autopsies, but to my knowledge they were performed by a charlatan that was willing to say anything - he didn't even have access to the body.