Completely agreed. That's exactly why I said "shattering from thermal shock is absurd, and the glass must be under some kind of stress from how it's secured to the phone" in the parent comment.
To further your explanation a little, the reason the outermost layer is put into compressive stress is that, when it's scratched, the stress will cause the crack to "close up" and so the glass doesn't shatter. Unfortunately, the interior of the glass is under tensile stress as a result, and so if the scratch happens to be deep enough to reach that tensile-stressed layer then the glass will shatter. Putting the glass in a case which alters this carefully managed stress state, i.e. by trying to attach it to a rim that's acting to pull it slightly apart, is what can cause this kind of failure.
But this is all speculation on my part, and even if true the reality might be more subtle.
Edit: I just found a good article from AnandTech that explains some of this. Take note of the first figure in particular.
Here is a more likely scenario (that my friend the materials engineer gave me):
Gorilla Glass is made by "stuffing" ions into the empty spaces so that the glass is under very high compressive stress. It's really known as "High ion exchange glass" This in turn, gives it very high compressive strength. However, it does have a significant downside that never gets mentioned: Shock absorption and scratches can cause later cracks and shattering. This is just a fact of life. When you place the outer layer under compressive strain, and then begin to compromise part of the outer layer, or begin to stress it, you can eventually cause release of those compressive forces. Once partially compromised, it may seem to be good, but can fail instantly.