Same thing started happening to my little yorkie Hermes
since yesterday. He yawned and his jaw started trembling. He kepts yawning and it would tremble for about six seconds. He is also 5 months and teething. I noticed that two of of his molars (on opposite sides) are getting pushed down by his incoming teeth. I called the vet in the morning and they said they'd get back to me. They told me it appears to be normal, but to keep an eye out for it. That if I feel like it's a problem to make an appt. so they can look into possible causes like neural issues.