while rare it is not uncommon for glass panels to fail due to stresss and perhaps some type of defect in the glass panel. It seems to happen mostly in hot conditions when the a/c is used in the vehicle, under certain conditions a glass panel will fail and create an explosion effect. It happened to my father-in-law on his Dodge Caravan, driving on the highway in summer heat with the a/c on, the rear quarter panel glass blew out, it scared the crap out of everyone in the vehicle as it sounded like someone shot out the glass with a gun. I've seen it happen on vehicles, home windows, balcony glass panels, etc.... . I hope your son wasn't too scared, it can be quite alarming when it happens. | 
