Internal pane spontaneously shattered.

[Notch7]

Very likely in fact I'll go as far and guarantee it was a nickel sulphide inclusion

Toughened safety glass, generally found in areas where the glass is below 800mm from the floor or within 300mm of a door but can be fitted anywhere, glass breaking on its own its quite common and especially this time of year, I've already seen it 3 times in the last 2 months, unfortunately toughened glass won't be cheap if your paying someone to do it

Yeah, when I ran my own joinery company I had these happen at least half a dozen times a year, mostly conservatory or orangeries not often windows or doors.

Its weird how they can go at any time, sometimes soon after installing sometimes years later