Sudden temperature changes, like hot water on cold glass, can cause stuff like that sometimes, I have seen windows break in cars from just sitting in the sun and getting too hot with the windows rolled up in the sun. Boom, there goes a windshield or back glass, its not something that happens a lot, but it does happen.
Rick Kilpatrick, AAS Automotive Systems Tech, Alamance Community College, Class of 99. 06 5 speed manual Mustang with Pony pkg : GT Mac Mufflers, true duals with one custom pipe. 2.5" system. GT bumper cover, all installed March 7&8 2006. AFE Stage 1 CAI, Motoblue Underdrive pulley, GT aluminum shifter knob, honeycomb rear blackout panel,and dash kit installed. Roush hood scoop and side window louvers installed. SS taillight trim has been installed, new alum. GT steering wheel installed.