I've heard of back glasses exploding(not on an HHR),usually in winter when the glass heater is used but I've never heard of spontaneous explosion of a side glass.Tempered glass is funny stuff-I saw a door glass explode on a workbench when the tech was installing a sash channel where heat had to be used.The glass was in the process of cooling after the sash was installed when it shattered.

 

It's possible like has been said before. The conditions where just right and the fissure in the glass let loose. I've been told that even a grain of porcelain that's sitting in the window felt can let loose a tempered glass window.

 

Ive had this happen while driving a Hyundai Elantra with the a sunroof. We were just cruiseing down the road and then heard a pop. Luckily the cover was closed or we would of had glass ever were. Also happened to my dad and some of his coworkers on there way to work in a company van. It was one of those big 12 passenger vans and just driving down the road the front passanger window just exploded.

 

It happen to my Grandfathers 1984 Delta 88 Sedan's rear glass. He was sick for a while (years ago) and hadn't used the car for a few weeks in august. He had called me and asked me to come over because someone smashed his glass. Had a dark brown interior and the car was parked in his driveway in direct sun all day, every day. When I looked at the car the rear glass was bulged/blown outward. Built up heat had to be the culprit. Strange by true.

 

This happens in El Paso alot too,,,,early in the year it gets hot and windy, between the heat and press changes it would pop the weakest window..always kept them cracked open..