I did pottery for several years, even on a small commercial scale.
The clay cracks when it does not dry evenly enough. As the clay dries, it shrinks. If it does not all shrink together, the piece pulls itself apart. This means if the exterior dries much faster than the interior, or if different sections dry at different rates, (think thick bottom and thin walls) it can definitely crack. Also, impurities can disrupt uniform drying as well.
Photos would definitely help.
