When you play as a vampire with the Witchery mod, you can take damage that would normally kill you, and not die. This results in the player's inventory getting put into a grave while they are still alive. The fix for this should be as simple as changing the event from null health to death.

Just some info on the ramifications of this. If you are a vampire and get stuck in something that does repetitive damage (such as a Hardcore Ender Expansion node exploding), you will generate a gravestone every time you are damaged. Depending on the rate of the damage, this could lead to 200+ gravestones being generated, all but one being empty.