When I put my mare in the round pen she will sometimes pin her ears and stare at me while she's trotting. It's not all the time, maybe once every couple of laps around the pen and it only lasts a few seconds. There isn't anything specific that triggers it.
I flick the whip at her and stand my ground when she does this and she will quit. However, I'm obviously not correcting her hard enough because she continues to try it.
She has good ground manners, she will respond to my voice ques in the pen even though she has an "ear fit" sometimes, she is not an aggressive horse by any means but this looks like a very aggressive action from her and I don't want to let it escalate to anything worse.