Through autoerotic asphyxiation, which is strange, but not uncommon. I have come across several deaths from this in various cases where a boy/man momentarily strangles himself while masturbating to heighten the pleasure of the climax.
It is generally classified as "Death by Misadventure."

Sadly, teens are the victims of this on occasion, but families (justifiably so) seldom reveal the indelicate details publicly.

I'm not saying that this is what happened to Carradine, but the circumstances are consistent with the practice.