Almost all cracks that happen in pottery are due to how it was made, dried, or fired. It can be from a poorly designed pot, using the wrong clay body for what you are trying to make, to issues with drying. They can be caused by the techniques that were used to form the pot, or from joining parts together at the wrong time. Cracks can also be from poor clay/glaze fit as well as how you stack and fire your pottery. Prevention is the key to fixing cracking problems and knowing what is causing your pots to crack will help you to avoid cracking in the future.