Pinned Ears when playing games

I've started working with one of the horses at the barn and I've noticed that when I ask him to do a game such as "Hide the Hiney", he pins his ears back but shows absolutely no aggression towards me. With all the other ground work we do this is the only game in which he does this. He is a very gentle horse and always does what I ask. I was just wondering why he is pinning his ears back and is it something I need to be concerned about? Also if it is a concern how can I teach him to not pin his ears back when playing games? Thanks for all your help!

Sounds like he's bored with that particular game, but is too much of a gentleman to do anything about it other than pin his ears.

Is he actually pinning them flat back or is he following your motion as you point or gesture to his rear end? If he's paying attention behind his hind his ears will turn back to hear better, but won't be pinned down flat like he's mad.

This is quite common so as long as he shows no signs of aggression don't worry about it.

He is pinning them flat back and all I do is run or walk to one side or the other and tilt my head and try to look at his hiney till he does hide his hiney.

Thank you so much! I had a feeling it was no issue since he shows no sign of aggression but I just wanted to make sure

Then he's probably telling you, "I got this, can we move on?"