The main reason an OOAK cracks, it is because the clay didn't cure enough when baking, you have to bake the whole thickness of the clay, this is based on time and temperature, this you learn as you go along, but if you change your oven then you have to trial it again, if the clay isn't cooked throughout the undercooked part underneath eats through the baked clay causing cracking, There are other reasons too, a knock if hard enough would weaken the clay, it would not always break at the time but could do later on, sometimes if you have an air bubble trapped in the clay this can cause moon shaped cracks, but they mostly apear at the time of baking, other people have said drastic changes of temperature can do this too, I hope this is of help to you,