Why would my dog do this?

I have the smartest, laid back big dog. She is a mixed breed, mostly German Shephard. She is a pound dog, and we adopted her when she was around two years old. This dog has NEVER gone pee or poo in the house. I have left her in many many times while I had to go out and she has never misbehaved in any way. Recently I got a new bedroom set, never slept in by anyone, and when I got home from an errand several days ago, she had gotten on the brand new bed and made the largest, smelliest deposit that you could imagine. I am baffeled as to why she would do this. Of course it was after the fact and too late to scold her, but I just cannot imagine why. Any ideas? Thanks