Possible cause of sudden breakage of windshield?

Question

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:

Answer 1

Glass breaks because of one reason: there is more stress on it than it can handle. Somewhere on the glass, there was a stress riser which formed. Wikipedia deems a stress riser (or stress concentration) as such:

A stress concentration (often called stress raisers or stress risers) is a location in an object where stress is concentrated. An object is strongest when force is evenly distributed over its area, so a reduction in area, e.g., caused by a crack, results in a localized increase in stress. A material can fail, via a propagating crack, when a concentrated stress exceeds the material's theoretical cohesive strength. The real fracture strength of a material is always lower than the theoretical value because most materials contain small cracks or contaminants (especially foreign particles) that concentrate stress. Fatigue cracks always start at stress raisers, so removing such defects increases the fatigue strength.

While without examining the windshield it would be hard to give the exact reason why it failed, obviously there was too much stress at some point in the windshield. This could have been caused by many different things. Something which happened previously may have setup the stress riser, then certain other factors may have caused it to fail after the fact. An example might be that your Father ran over a pot-hole which jarred the frame of the vehicle in just the right direction, which tweaked the windshield just enough. Then a bug hits the windshield and BANG, the windshield shatters. (I'm over exaggerating here, but you get the idea). Once that stress riser is setup, any little thing can make it happen. It could be that the glue used to set the windshield was a little thin in one area, which allowed metal from the windshield frame to make contact with the glass, thus setting up the stress point.

I could go on with an endless array of reasons. Hopefully I've given you some idea as to the why it could happen, though I doubt I have given you the exact reason why it did happen.