well in acting it is pretty hard to cry at will, sometimes one of your tear ducts will respond better than the other and once one of them has the actor might just think that that will be enough, when I yawn my eyes get really watery (people actually think I'm crying) but when I blink right after a yawn only one of the watery eyes releases a tear, I yawn again and do the same, the other eye usually follows

I'm not saying this is what happens, this is just a guess for me