I have four geldings. While they will yawn if I come in the barn unexpectedly and turn the lights on (waking them up), the only time any of them yawn (as I have learned) is if they are in some sort of discomfort.
While it is ulcers with my Arab, it doesn't have to be limited to ulcers. It can be anything.
I have one horse who foundered pretty bad and ended up with torn ligaments from a trim gone bad. He did a lot of yawning until he healed.
As far as only yawning when she sees the bridle coming at her, that could be a learned response from an old issue.
Maybe she had tooth issues that went un-noticed and the bridle bothered her.
Maybe she has something wrong in the poll area to where the bridle makes her uncomfortable.
Or maybe she's sandbagging you so you won't put the bridle on her.
Are you able to contact the previous owners to ask them if she did this with them?
If this behavior is consistent, the only way to know what's up with her is to have her examined