Roger, It is my estimation that the shower door glass was not tempered correctly. If it was, it would not have broken into sharp pieces. Or for that matter, wouldn't have failed in the first place.
Improperly tempered glass will shatter from microscopic cracks made from the production process. There is no way to tell by just looking at the glass if it is good or not. Also, there is no way to tell when it will fail. The spontaneous failure you experienced is not unusual for incorrectly tempered glass.
If both doors were made from the same lot, I would not trust the replacement either.
I would demand that the manufacturer follow up with their glass supplier for an explanation.