It's likely just thermal shock.
The way tempered glass is manufactured, it is under a very high (but equalized) level of stress as it sits. That straining against itself makes it extremely strong, but if anything unbalances it too much, whether a mechanical impact or thermal shock, it will rip itself apart in a catastrophic manner.
In a related but opposite sort of scenario, I had a ton of snow on my roof and went to work, but it was a sunny & warm day, at least in the direct sunlight. It warmed the interior of my T4R up pretty high over the course of the day, and the snow on my roof melted and the blanket of snow started to slide down over the top of my windshield. I caught it in the act a few hours later, the top several inches of my windshield (which wasn't all that new, but was in pretty good shape) had numerous cracks.
This sort of thing just happens when cold is focused on a tiny area of warm glass, or heat is focused on a tiny area of cold glass. Or, quite possibly if a defroster warms cold glass up to melt the ice, but there's a little spot where a more massive chunk of snow or ice is pressed against the heated glass.
To the folks who had the glass pop while the defroster was on, is there any chance there was a mass of snow or ice piled-up on the rear wiper blade and in contact with the glass? If not, I'd guess it was just a defect with the defroster filament causing the glass to heat unevenly, or maybe a weak spot defect in the glass that was aggravated by less of a thermal shock than the normal window should've been able to withstand. Either way, the problem is likely solved by replacing the window, so long as it's not a pervasive defect.
Certain models from just about every manufacturer have issues with their moonroofs. Just hit Google with "exploding moonroof" for many, many tales of woe.