It started at a sharp angled indent on the outside (that coincided with a thin wall section) and grew around the perimeter (not visible). From there it branched to the base.
Dunting and Cracking of Clay Bodies During Firing
Ceramic industry can fire much faster and deal with much heavier objects than potters can, how do they do it. The answer is they pay more attention to the basics, it is all common sense and good equipment.