Thanks for all of the answers! She is just 4 and straight off the track, so she either just does it with me, or learned it at the track. She had a baby tooth until a few weeks ago. She got her teeth done and the guy said that since all of her baby teeth were gone, he could even out all of her teeth. She starts yawning the second I'm done with the girth. I don't see her yawn other times, so I guess I can rule out ulcers?
It could also be her stretching her jaw like you posted Vidaloco. She likes streching, often stretching her front legs out and bowing like a dog.
I guess since I know it's not teeth and it's likely not ulcers, I can rule out discomfort and just say it's a quirk of hers? Anyway- It makes putting the bridle on easy- Just slip the bit in while her mouth is open :)