There is a small container called lachrymal sac at the top of your eyes. The lachrymal sac is full of tears, and there is a tube at the corner of our eye which link to the nose. When you yawn, the muscles will squeeze the sac and your tears will spill out. In addition to that, when you feel tired, your breathing becomes more shallow. It is the body's way of assimilating more oxygen. However, you should not take it necessary, tearing is normal because it protects your eyes and there is no way to stop the teary eyes. It is good to your body!