Im with Shotgun Jones. It appears to be a detonation type failure. I will guess that the primer firing failed to inginte the powder charge and only moved the shotcharge about an inch ahead and the crimp partially opened holding the payload in the forcing cone, then the powder charge finally combusted and its pressure wave hit that partially advanced shotcharge whuich became an obstruction and it let go.

Brian
LTC, USA Ret.
NRA Patron Member
AHFGCA Life Member
USPSA Life Member