What would cause the back window to implode?
Rear and side windows of all vehicles sold in the United States are made of "tempered glass". Tempered glass is used because it has a desirable characteristic in that it does not create long, sharp shards when it breaks. Tempered glass breaks into crystal shapes, with far less dangerous edges. You can still be cut by broken tempered glass, but not the deep, dangerous cuts that threatens life. When glass is tempered it must first be formed into the final shape. Tempered glass cannot be cut, and even slight bending will cause the entire sheet to shatter into the characteristic small crystals. If the vehicle is heated unevenly and the vehicle is attempting bend the glass, it will shatter. As a result, auto manufactures make sure that the vehicles do not put those types of stresses on the glass. Typically, when tempered auto glass shatters, the glass has been damaged through scratches or chips. After that, even moderate temperature change can cause the tempered glass to shatter. The scratches or chips don't even need to be very severe.