For some reason, the stretching of jaw and face muscles is necessary for a good yawn. It is possible that yawns are contagious because at one time in evolutionary history, the yawn served to coordinate the social behavior of a group of animals. When one member of the group yawned to signal an event, all the other members of the group also yawned. None of this has been proven true and yawns are still one of the mysteries of the mind. Try an experiment to keep track of your own yawning.
Pandiculation is the act of stretching and yawning.
Bulletin Psychonomic Soc. Bulletin Psychonomic Soc. R.R. Yawning as a sterotyped action pattern and releasing stimulus.