When you see someone yawn, you might yawn too and something else may also happen. You might shed a tear or two.
When we yawn, the facial muscles surrounding our eyes pull tight. This may put pressure on our lacrimal glands (the glands that are neatly tucked away deep beneath our upper eyelids just below our eyebrow bones.) These glands produce the watery component to our eyes' own natural tears. They are working to produce and release tears slowly throughout the day to coat the surface of our eyes at all times, not just when we cry (think about it, that is why our eyes always look so glossy.) When the facial muscles tighten during a yawn, the lacrimal glands may get "squeezed" causing them to release a small amount of tears that they were storing to release later.
So the next time you let out a big yawn (hopefully it didn't happen already while reading this), pay attention to the muscles around your eyes to feel them tighten and then make note whether or not you get a little teary eyed. It's not sad, it's just science.