It is because the glass raw material contains nickel sulfide crystals. This kind of nickel sulfide crystal has two forms: A and B. Under specific time and temperature conditions, form A will change to B. At the same time, the volume of form B is 3-5 times the volume of form A.
So although the volume of this crystal is minimal, if it becomes B-shape, the increase in volume will affect the internal compressive stress balance of the whole glass piece and cause it to break.
Unfortunately, it can’t avoid tempered glass self-exploding entirely under the current technology. Read more from our post: What is tempered glass?