where do you cross tie her in the barn? In the aisle with any doorways or open windows near? I would hedge a bet that she is just listening to something behind her. In my experience, if a horse pins his ears, you usually get more than just pinned ears a very short moment later. Kicking, biting, stomping, etc. the pinned ears are your warning sign.
If she isn't edgy, and doesn't seem upset about anything you are doing, then I think she's just 'tuned in' to whatever may be behind her or she could be really relaxed. Horses ears are never 'at attention' and forward and perky all the time.