Likely all of these and even more (exertion, lingering Covid symptoms, etc). So, so much affects the heart and lungs. With Mr. Floyd though, his interaction with police becomes a lethal game of dominoes imo. Drugs could have increased heart rate and possibly also lowered respirations; anxiety and/or panic could have increased heart rate and made him feel short of breath; coronary artery disease can cause shortness of breath; prone restraint and weight on his back didn't allow his lungs to expand or filter carbon dioxide; and a rush of adrenalin, cortisol and other stress hormones could have triggered an increase of heart rate too which may have increased his breathlessness. All that time, for all the reasons above and some we may never know, his heart needed more oxygen than it could possibly receive.
I'm not trained in any speciality but I see anxiety with Mr. Floyd because I suffer from it myself. The more nervous, anxious and panicked you become, the more difficult it becomes to breathe. I'd expect a long-time officer to recognize potential symptoms of a panic attack and not exacerbate such an attack but jmo and fwiw.
Not sure many people understand just how literal this is. While in hospital I stopped breathing twice. I went from conscious, breathing and even speaking to being put on life support just minutes later.
(On the bright side, I am now rocking a pretty badass tracheostomy scar in the shape of a heart. Silver linings and all.

)