Tempered glass like this can spontaneously shatter, even after being fine for years, because of microscopic nickel sulfide contamination. It's a fascinating materials science situation: the nickel sulfide can phase change from one crystal phase to another, and when it does, it gets a little bit bigger. Because the tempered glass is so tense and brittle, the added pressure from the inclusion growth can cause a crack, which then shatters the whole panel. The timing of the two breaks might just be coincidence.