What is known/taught about yawning during meditation?
There are a whole host of phenomenon that happen at the early stages of meditation.  Some others you might experience are a feeling that your face is swelling, that your sinuses are filled with cotton, your muscles might twitch involuntary, it may feel like your hands or other parts of your body are disappearing, that your body is being twisted, that someone is messing with your hair, and so on and so forth.  None of these mean anything, but they are a good indication that you are just touching the edges of samadhi.  As your concentration deepens, all of these phenomenon will ultimately disappear.