Tempered glass, the type which is used for stuff like shower screens or car windows is under constant tension against itself. The inside of the glass wants to be bigger than the outside. Normally this makes it harder to break as the bonds on the outside are so strong and uniform they prevent it from bending or shearing as easily as normal glass.

The problem is that if you manage to break those bonds, the outside can very rapidly tear itself apart as it finds a way to relieve that tension. It basically pops like a balloon. The reason it breaks into small pieces is because anything bigger is still under tension and once the surface is damaged the crack will spread to ease it.

So your glass could have had a defect or damage which broke a few of those bonds, and heat cycles eventually pushed it to break the next bond along and start a chain reaction which destroyed the whole thing.

Edit: it could even be that some grit fell in the housing or something. The reason spark plugs break car windows is because the edges are sharp enough to cut the glass with very little force.