My DD13 did the same thing for a while when she was younger. She still does it sometimes for fun, she knew that she was writing upside down and backwards and thought it was more interesting to do so than to write the correct way. It also allowed her to write this way to my mom, who used to send her letters written upside down and backwards in cursive.
This eventually lead to her being able to read upside down as well. A skill that is also pretty rare, but very helpful when having to share a book in school or when helping others out who sit behind you.
