What causes extremities to tremor when yawning?

For example the left or right hand shakes somewhat violently during a yawn to the point where you can't hold a mouse, drinks are spilled, etc.

I got curious about it this morning, and it seems to be something that's fairly common (Google even autocompletes it for you). But there doesn't seem to be much information on it on the internet, which seems like something only AskScience can remedy.