Well, I don't know for sure, but I heard that excess yawning in people has been linked to regulating body temp. When we get a little too warm or whatever, we yawn as the sudden influx of air does something to cool us off.
Have no idea of that's true or anything, but if so, it could apply to horses,I suppose. Or...you just have a bored horse.
Actually I was told today it might be due to her selenium deficiency. It could be that or maybe I'm rubbing her in a spot she likes. Could be either way cause I only notice it when I take her into the barn to groom her or when I'm petting her.
I found this post amusing. Looks like you have a happy, relaxed horse to me I haven't heard of this before. That's quite funny. If you are concerned you should give your local vet a call. I'm sure they will be more than happy to answer your question