From the sounds of it, I would guess medical issue, not clear if he died due to crash or was dying before the crash, e.g. large stroke or 'heart attack' caused crash and impact forces largely incidental. I hope the ME/pathologists asks the family permission for an autopsy.

If throttle stuck could always pull mix, ground mags, or go for emergency fuel shutoff if an alert, regular person. It just doesn't sound normal at all.