Yawning is a reflex of long deep inspiration with mouth wide open followed by slow expiration. As we yawn, the facial muscles around the eyes are pulled tight which put pressure on our lacrimal glands. Lacrimal glands are small glands located behind the outer part of each upper lead that secrete the watery component of tears.
Yawning often increases the secretion of tears because of muscular tension on the lacrimal glands, which may simultaneously dam up the drainage system. When the facial muscles tighten during a yawn, the lacrimal glands may get squeezed causing release of small amount of tears.
The involuntary act of yawning usually includes opening the mouth very wide while slowly taking in a deep breath. In addition, the eyes often close as well. The result is a temporary rearrangement of the facial musculature. Repeated yawning can multiply this tension.
The tear system eventually leads to the nasolacrimal canal, linking the eyes to the nose. Thus , episodes of yawning can produce a runny nose also.