I had the rear glass sliding door break on my PPOR a couple months ago. The ppor about 18months since finished construction.
I don't know how it happened either. I asked the glazier who fixed it, he said he fixes half a dozen a week just the same. Its the poor quality glass cheap builders use that develops a pressure point and just shatters from there. He put better quality glass back in.
If there's no sign of foul play or break & enter, I'd say it'd come out of your building insurance.