The "Ask a Mortician " Youtube channel gives a fascinating explanation of why exploding caskets are a thing. Sometimes, when a person is put inside of a casket in an above ground mausoleum, the built up gasses from their dead body will lead to an "explosion". The lid of the casket can pop off, sometimes dislodging the marble front of the crypt as well. After learning this, I've officially decided I would like to be cremated when I die.
"In the airless environment of the sealer casket, it's the anaerobic bacteria that thrive. Unlike their oxygen-fueled aerobic counterparts, these agents attack the body's organic matter by putrefying it, turning soft body parts to mush and bloating the corpse with foul-smelling gas. In entombment in the aboveground mausoleum, the buildup of methane gas has been sufficient in some cases to blow the lid off caskets and marble door panels off crypts. "
