OK, this is a mistake:

So notice how the AT-AT head is facing 90 degrees from the body, a feet I’m pretty sure it cannot achieve. Its also odd considering that in the prior shot it was facing directly forward. Something also seems off with the perspective as the head is behind the near tree, even though the tree appears from the Arial shot to be some distance away.
I assume this came about because Lucas didn’t feel people could understand it was the AT-AT form a profile shot(?), but I really don’t know.