Originally Posted by Freebase Dali

Maybe after internal pressures built up due to decomposition, but unless the bottle was being held in place by muscle tension, I see no reason why it'd come out immediately upon death.

Death doesn't just cause things to come flying out of your ass. When you die, any muscle control you used to have will have failed, so naturally, if you were restricting a bowel movement or urination, with the help of internal vs. external pressures and gravity, you will probably leak.
But if there is basic equalization of pressure before muscle control is lost, don't expect fireworks.