Why does yawning trigger tears?
When you yawn, the muscles surrounding the glands which are responsible for bringing tears, i.e. the Lacrimal glands get squeezed leading to teary eyes. This is the location of Lacrimal gland (it's present at the top left corner in this image):

https://upload.wikimedia.org/wikipedia/commons/c/cf/Gray896.png



These glands actually work all the time as they've the responsibility to coat the surface of our eyes (that's the reason our eyes appear glistening). When they get squeezed, due to the tightening of facial muscles surrounding the eye, or maybe because your closing eyelids too tightly while yawning, they release a small amount of "tears" they would have otherwise used at a later moment.



Also, as a side note, there are more than one type of tears, this specific type of tears are called reflex tears. Quoting a Wikipedia article as a reference (https://en.wikipedia.org/wiki/Tears#Types):


  $\ldots$ It can also occur with bright light and hot or peppery stimuli to the tongue and mouth. It is also linked with vomiting, coughing and yawning. These reflex tears attempt to wash out irritants that may have come into contact with the eye.