Long answer, short... it doesn't matter, it's just kinda funny.

A slightly longer answer... all things equal, may as well go with the artist's original vision. Usually, the hardware on the back of the work dictates how you hang it, so it could have been an error at the framers or some random assistant, or an art handler, but if the artist didn't put any hardware or other indication of orientation then it is likely not vital to the purpose of the work.