Random horse grooming questions?

OK so i have a random question, why is it that every time when we try to groom our horse he gets a bit "too excited" i would say, and starts to open and close his mouth to where it looks like he is yawning, he stays still and everything. He is around 3 years old, not trained just used to basic handling, and has not been castrated yet. I'm just wondering is that normal behavior?