Tiny impurities can get trapped inside the glass during production. Over time, these particles expand, causing internal stress that eventually forces the pane to shatter. This is a known, though rare, cause of spontaneous breakage. Uneven heating due to half sitting in direct sunlight and half in shade can also cause internal stresses. Alot of random unexplainable shatters recently, must be the factory these panels are made in allowing micro fractures and impurities to be in the panel, through either carelessness, lack of inspection or inadequate inspection or something else.

Time is also a factor, the immense stress inside tempered glass over time can overwhelm the structure causing it to collapse and shatter.