My father was driving at about 30/40 kmph with ambient temperature of about 30 degree Celsius, AC turned off, when the windshield shattered. I want to know the possible causes. I have nullified all the causes I could think of:

1. some minor stone/rock hitting the screen
2. sudden change in ambient/internal temperatures

Facts:

1. When I got it replaced it turned out that it was not original factory fitted, i.e. had been replaced once.
2. It was of the type which shatters immediately(Tempered i assume) as opposed to the one which cracks(Laminated).