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. :-)
There's no need for toxic cookware. Get high quality cast iron and you can give it to your future generations like my grandmother did for me. And if that flakes into your food, it's just iron which your body needs!
But Big Bird is okay in the kitchen? ;-)
Smoke inhalation and Teflon toxicity are really different death mechanisms -- if you're interested, a quick read of the abstracts of articles on mass bird deaths from PTFE toxicity at http://www.ncbi.nlm.nih.gov/pubmed/10879927 or http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3276392/ is illuminating.
Butter in cast iron is toxic at 260 degrees. Teflon isn't toxic until 280 degrees.
Check out http://www.ncbi.nlm.nih.gov/pubmed/4723395
> Abstract: Japanese quail and parakeets were exposed to the pyrolysis products of: (1) commercial fry pans coated with Teflon® finish, (2) plastic fry pan handles, (3) corn oil, and (4) butter. Other materials used in the test were commercial aluminum fry pans and cast iron fry pans. A four-hour Approximate Lethal Temperature (ALT) was determined for each test material for each specie. All products tested were lethal to Japanese quail with ALT's ranging from 260° C for butter in cast iron to 370 °C for a plastic handle in a glass container. The ALT for a fry pan coated with Teflon® finish was 330°C. All products were also lethal to parakeets with ALT's of 260°C for both the pan coated with Teflon® finish plus butter and the cast iron pan plus butter. The ALT for a fry pan coated with Teflon® finish was 280°C.
Really? "Real" nonstick pans (not walmart) sell for something like $40 to $100+ a piece, but cast iron (Lodge Inc, etc) sells for something like $20. So CI works better, last forever, is cheaper, and its less toxic.
Its true that you can buy a pan shaped object from walmart for $5 that doesn't really work, but in all fairness the CI equivalent is the "free to a good home" pile of CI at an estate sale, so CI is still cheaper.
I will say there is one, exactly one use for teflon pans and thats burned cheese or burned eggs. Just something about it chemically that makes it stick to cast iron. Yet meats don't stick, very strange.