It doesn't for everyone, but for some people it pinches their tear glands and squeezes out tear drops.

For others it's because the squint so hard when the yawn that they sqeeze out tears by pressing their eyes against the eye sockets.
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It is normal for your eyes to tear up when you yawn and heres why....
Tears drain away from each eye along two channels marked by a tiny opening at the inner edge of each lid. The channels lead to the lacrimal sac at the side of the nose, which is why your nose runs when you cry. Yawning exerts pressure on the lacrimal sac causing normal tearing.There are 2 explanations for why this happens. The first is 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. And then there is a more simplistic reason which is 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.
i hope this helps:)
Source(s):
www.askville.amazon.com

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