Vehicle glass can shatter a while after an impact - doesn't have to shatter straight away. An example was when we were in a Honda Civic in a line of traffic - a council contractor was cutting grass at the side of the road and the mower threw up a stone which hit the passenger side window. Fully five minutes later, and several hundred yards away from the worker, the glass shattered.
So unless there is a manufacturing defect, it probably won't be covered under warranty. Have you checked the glass replacement clause of your insurance? Be aware that not all policies cover glass roofs.