Extremes of temperature difference, such as having very hot air just under the glass and rapidly cooling air just above, are thought to be the cause of this. The dealers will say it was a stone....

This has been happening recently to a number of brands of cars, not just Audi. Consumer reports had an article on it a few months ago. Glass is extremely strong until it gets a defect such as a scratch at which point the strength does decrease dramatically. The tempered glass used on side windows and sunroofs also has built in stress so when it goes it explodes. It would not surprise me if the reason we are hearing about more of these is that the car makers are using thinner glass to save weight. There have been recommendations that the sunroofs be made like the windshield with two layers of glass with a tough layer of plastic film bonded between them.