[villagetinker]

I am going to guess a lawn mower hit a stone which then hit your glass panel. The spider web of broken glass suggests (IMHO) a hit on the right side of the panel about halfway up. Tempered glass is designed to shatter just like this. I am guessing id you look very carefully on the outside you will find a small (vary small) ding in the glass and this is what caused the pane to shatter.