There are two possible explanations:
1- During a yawn, there is a slight increase in pressure inside the skull (likely caused by slight increases in the amount of oxygen taken in during a deep yawn). This causes the cerebrospinal fluid (CSF), the fluid that surrounds the brain and flows through the brain's ventricle system, to be forced out faster than normal. That fluid enters the lachrymal (tear) system. That increase in fluid pushes out the tears.
2 - the more simplistic reason may be that when you yawn you scrunch up a bunch of facial muscles. That causes the tear glands and ducts to be squished, forcing out some tears.www.answerbag.com/q_view/15528#ixzz1Cgt8MgMT