Been there done that... I have an 05 Wrangler X and one cold day (like 3 degrees F) I started it up and put on the defroster. I came back out 10 minutes later to leave for work and there were two cracks starting at the bottom and extending upwards and outwards. They originated right in the center of the defroster grille. By the time I got to work they had grown to the middle of the glass (of course, that's what they do). The glass guy who changed it said it's fairly common to crack a windshield with the defroster. No rocks, no dings, no prior damage. I blame the design. Windshields are not supposed to be glued in place. They need some flexibility. The right way to install a windsield is like on a CJ, with a rubber gasket.
By the way, the replacement glass I got installed is NOT flat. For some reason it's bubbled out a little at the sides. You can't notice it from the inside unless light reflects on it the right way. It sure looks strange from the outside, though. If you stand at the side of the Jeep and look across the windshield you can see how bad it is. Make sure they use an OEM glass (or at least a flat one!!). Curved glass will destroy the precision aerodynamics that were carefully designed into the Wrangler. Best of luck - John