Houston we have a problem

We woke up to this not so pleasant surprise on Sunday morning. A crack in my tailgate, a nice big long crack! We haven't backed into anything, so how could a car just crack? Robbie says from the cold we had and then putting it in the heated garage, I'm not believing that because it wasn't really ALL that cold. I've had it since new in 2003 and we've had extreme cold weather for longer periods of time and kept the car in the garage all the time then. I'm thinking somehow it has to do with when the school bus backed into me. But Robbie says that's really far fetched. My poor baby, she'll have to go get some repair work again. Just when you think you're going to be ahead, something always comes up!!