Carradine was found by a maid hanging in the closet with a rope tied around his neck and groin. He may have had a heart attack. The police originally thought it was suicide but Carradine’s friends disputed that theory since he was there to film a movie and had other work lined up:

Police Lt. Teerapop Luanseng had said on Thursday that Carradine’s body was found “naked, hanging in a closet,” and that police at the time suspected suicide.
But one of Carradine’s managers, Tiffany Smith of Binder & Associates, dismissed the theory.
“All we can say is, we know David would never have committed suicide,” said Tiffany Smith, of Binder & Associates, his management company. “We’re just waiting for them to finish the investigation and find out what really happened. He really appreciated everything life has to give … and that’s not something David would ever do to himself.”
Carradine had flown to Thailand last week and began work on a film titled “Stretch” two days before his death, Smith said. He had several other projects lined up after the action film, which was being directed by Charles De Meaux.

Very strange! We will probably never know what happened.
BTW, one of Carradine’s last films was called “My Suicide” about a 17 year-old-boy who causes a stir when he announces on film (for a class project) that he’s going to kill himself.