Why does glass spontaneously shatter?
I'm 95% confident that, in this case, it was nickel sulfide contaminant.  I took photos of the epicenter of the shatter and zoomed in on the computer.  The contaminant is pretty clear to see:

This is pretty much exactly what articles like these show: https://www.constructionspecifier.com/fully-tempered-glass-and-spontaneous-fracture/2/