Apple doesn’t have a branded screen protector. But regardless…a screen protector and case don’t guarantee cracking won’t happen but typically lessens the chance. The right circumstances of a drop or impact could still cause it to happen though. It’s hard to say what may have happened in your case but barring an issue where a true defect occurred that lead to a crack and shattering occurring it is likely that the damage was caused by a drop, some sort of impact, pressure (such as an ill-fitting case or something heavy putting force and pressure on the device) or something along those lines was the culprit. You could always have the device inspected by Apple to rule out anything else but otherwise know that the warranty does not cover accidental damage. AppleCare+ does if you have it