Carrier/rack problems of the 2007 Toyota Matrix

I was driving home on a 2 lane city street a few weeks ago when I heard a loud crash/explosion of some kind on what seemed to be the top of my car, when I had gotten home I saw the sunroof had completely smashed as if something had fallen on top of it, but I and the guy who replaced the sunroof found no rocks or any point of impact on the sunroof, so it had appeared to crack spontaneously. We had been having icy weather the week prior but it had been cold enough for a while that a sudden temperature change couldn't have been what caused it. No glass fell into the vehicle because the inner lining of the roof had been pulled shut, but it cost $700 to replace it and we had to drive to omaha, 60 miles away from where I am, to get it replaced.