Well, yawning equalizes air pressure in your inner ear (let's say decreasing the pressure). If I decrease the pressure inside my head, Boyle's Law dictates that

Well, yawning equalizes air pressure in your inner ear (let's say decreasing the pressure). If I decrease the pressure inside my head, Boyle's Law dictates that must increases the pressure outside of my head (ie, the entire earth's atmosphere). Which means everybody around me now needs to equalize the air pressure in their head. So they yawn. Duh!