Wait.....it was still on the tracks, but wheels up......that sounds like somebody pick-it up and flip-it over..... I don't think I have ever seen that happen on its own. Anyways, I wish I could help you, but I don't have an Annie.
Edit: wow, my mind read "on its back" not the "at the back" that was writen. My bad.
Edit: wow, my mind read "on its back" not the "at the back" that was writen. My bad.