What causes an oven door to shatter?

Our oven door shattered just before Christmas, with what seems to be no reason at all… We were only cooking garlic bread… We had a new door for it ordered, and we just got it back yesterday, but today was the first time we used it again and it just shattered again and we were only cooking oven baked fries..

It was being used both times at roughly 220*C, and for some reason it was 2 minutes before the cooking time for both finished..

It wasn’t cold out either time, and nothing hit it or touched it, and it’s not old, it’s like a 3 year old oven and it’s not used that often…

I was just wondering if anyone would know of a reason as to why it would have shattered??