This fault is usually caused by a glaze that is too small for the clay body. The glaze contracts more than the clay body as they cool from their maturation temperature in the kiln. The resulting tension in the glaze causes it to crack.
Crazing may also be caused by overfiring a pot. This melts the silica in the clay body, which in turn changes its coefficient of expansion. That change can be enough to cause crazing.
Another potential reason for crazing can happen after the ware has been put into use. If the clay body is porous and should there be any unglazed areas, the fired clay body may absorb enough moisture to expand slightly. That expansion can be enough to upset the glaze-clay body fit and cause the glaze to craze.
A last, common cause for crazing is thermal shock. When the ware is subjected to abrupt and significant temperature changes, such as removing pottery from a kiln that is too hot, glazes are apt to craze. In addition, the clay body is almost always weakened as well.