I think twice before I have read about this on the Yahoo group long ago. No idea about the cause. Unless you have glass coverage or a really low insurance deductible, you are probably on the hook.
Auto glass shops may offer better pricing than Toyota. As a point of reference I have priced an aftermarket windshield at $242. Try to get defroster lines, if possible. Glass bits in the rear deck speakers buzz and are supposedly hard to remove. Sorry to not have any better news.
There are three primary sources of spontaneous glass breakage in cars:
1) Frame rubbing, pinching or warping causing localized stress on the glass. This can be compounded by thermal stresses.
2) Stress risers such as heavy scratches, rock dings, or even fabrication micro-fractures (called "checks" in the glass industry).
3) Nickel sulfide stone (a type of glass defect).
Since it was an older Prius, it is probaby either a bad scratch, chip, or rock ding in the glass, (with a remote possibility of a nickel sulfide stone). The failure of tempered glass appears almost explosive, with the glass turning to tiny fragments instantly.
A coworker had this happen to her on the way to work over a year ago. She had a Mercury minivan. It was also toward the end of fall, beginnning of winter and rather cold out that morning. It was totally unexpected. She was just driving along and then BANG; no back window.
Her husband and son came by to clean up the glass and put a cover over the window until they could get a replacement. The same thing probably occured, the window had a flaw and temperature differential between the outside and inside along with time caused it to finally give way. But it basicaly did "explode" as tempered glass will when it brakes.
Possibly a bit off topic, but I had that happen on a train I was riding in when I was 11 years old. I was looking out the window as we were coming into the station when suddenly this pattern of lines appeared on the glass. There was just enough time for me to turn my head before the glass exploded inward, as others have said will happen. It seems somebody threw a rock at the train. The neat thing was this guy jumping up and yelling, "Sabotage!" I remember it like it was yesterday. Fortunately the glass did its job and I wasn't hurt, but I did have to go to the hospital for them to check. And you don't know fast until you go to the emergency room with a possible eye injury. Probably being a little kid helped at the time too.
About 10 years ago a boy in a school parking lot was tossing rocks and hit my car's windshield. He came in and told me about it. We both examined the windshield and could only find a very tiny chip. Four hours later, I got in my car, shut the door and the windshield shattered.
Windshields are designed differently than the other windows. The other windows are tempered glass. Molecularly, the outside surfaces are tighter than the middle, because the outside is cooled quickly before the middle has a chance to cool. It is thus under its own tension, like a rubber band. But all you need do to break that tension is scratch the surface.
The windshield however is glass laminated plastic so that if the windshield does crack or shatter, it stays pretty much together.