Most often cracked plugs are from installation. It is far more common with Champion plugs due to their more brittle porcelain and thinner internal portion of the center wire.
Although installation isn't the only thing that can crack plugs, it is by far the most common, so look at your spark plug socket (it should have a rubber protector that also holds the plug it the socket) and review your technique so that you don't allow the socket to get cocked to the side as you finish tightening the plug. Also make sure you are not overtightening. If in doubt, use a torque wrench a few times to help gain the proper feel.
