One of my female buttons started doing something similar a few weeks ago. The "yawning" isn't constant, but it is extremely frequent. She also frequently opens her beak as she breathes, and occasionally (perhaps once per day or sometimes only every few days) makes wheezing sounds, that resemble a sneeze but coming from her chest. These sounds can happen once or go on for 10 minutes.
I took my button to a bird specialist (I happen to live near my state university's veterinary hospital, so the quality of care is phenomenal), who checked out everything he could think of, but in the end he had no idea what was causing the behavior. His best guess was that something internal is deformed due to bad genetics (my birds were rescues and their lives before I got them were not good to say the least). The vet didn't know what could have triggered it, but said it could be anything. It could also be a foreign object lodged somewhere, but he could find no evidence. Because of her size, x-rays would be extremely risky so I elected not to try. So far, she's doing just fine in spite of this. I watch her carefully, but her behavior has otherwise not changed. I'm still hopeful that somehow it will stop as suddenly as it started.
Good luck to you with your little bird. Maybe your scenario is different from mine, and something can be done. In my case, I had thought at first it was a crop issue, which the vet later disproved. Taking her to the vet was expensive, but it gave me peace of mind that I was doing everything I could and not letting her suffer over something I could fix. Looking into whether your case might be a crop problem would be a very good idea. I'm far from an expert, but I did a lot of research before taking mine to the vet, and what you describe sounds very similar to what I read about crop issues.
