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 (Like you turn on heater with snow on the outside of windshield)

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).
 

Edit: To make it clear, I'm adding an image, mine broke like the glass on the left:

![enter image description here][1]


  [1]: https://i.sstatic.net/lNYTy.png