Spontaneous glass breakage
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.