I do not know but will hypothesize that severe pressures were sometimes experienced. Perhaps the solid walls of the case did not cushion pressures and/or maybe the flat internal base of the hull created problems. In drawn hulls the mouth of the primer is well above the hull's base. Internal ballistics absolutely baffle me and weird things happen when any one ingredient is changed.

When an old man dies a library burns to the ground. (Old African proverb)