What would cause a hot surface ignitor to explode?

When not in use, the combustion chamber of your heating system is subjected to cooler temperatures. This causes contraction of parts and pieces. As electricity is applied to your igniter, a tremendous amount of heat and expansion is generated. The material used in igniters is mostly ceramic to help withstand the heat. Over time, the expansion and contraction takes it's toll on the igniter and causes failure. There may be a new type of igniter available for your system using Silicon Nitride, a much more durable material. Contact your manufacturer and see if they can get you a part number for the upgraded igniter. If not, you are stuck with the same old issue. Keep a spare on hand!