Poor quality control in manufacturing, natural crystalline formation defects, excessive impacts in handling, extreme thermal stresses, etc.
Cast iron, being a non-ductile material, in the daily thermal cycling of cooking, any minute microscopic fissure will start to propagate through a process known as metal fatigue, and eventually fail catastrophically.
To some degree, cast iron can be annealed or hardened, depending on your cooking habits. Something I do which I would not recommend to anyone else: I buy cheap cast iron cookware, heat it up red hot and pour cold water in it. If it didn't crack, them I heat it up again in an oven and let it cool down as slowly as possible to anneal any internal stress.
In any case, if the skillet is of great value to you, there are places that actually can repair it for you.
As a last tip, stop practicing skillet abuse and trying to be an iron chef. :-)