Read Dr. Timothy McCall's response:
Many people in the modern world, including many who practice yoga, live stressful lives and walk around most of the time in a state of sympathetic dominance. By that I mean that the sympathetic branch of the autonomic nervous system (ANS), associated with the "fight or flight" response, is more activated that the parasympathetic nervous system, the branch of the ANS associated with rest and relaxation.
Since stress hormones are stimulating, these people may not notice how tired they really are until they get into a situation in which they start to relax. Yogic breathing—such as gentle Ujjayi Pranayama (Victorious Breath)—or practices that lengthen the exhalation relative to the inhalation can pretty quickly lead to parasympathetic dominance, and that's probably why the yawning occurs. And, although we don't know why, yawning also seems to be contagious—once one student starts, it may spread like wildfire. As with students who fall asleep in Savasana (Corpse Pose), yawning may signal a sleep deficit that your students would be wise to address before health problems result.