Yes, it is normal. It happens to me all the time that my eyes get watery when I yawn. The production of the tears is great for our eyes as it can keep our eyes moist avoiding the eye health hazards caused by eye dryness. At first, you can search the wikipedia to know exactly how tears are made, and then their function and circulation. The tear fluids are created in small glands around your eyes and flow down tiny channels so that once they are irritated, they will produce tears to clean your eyes and lubricate your eyeballs. So we can assume that when we yawn, a lot of air pull into our body, and make the existing water evaporated. Further, the tiny channels producing the tears are too sensitive to such small moisture changes in our body. As a result, excessive tearing are going. Anyway, it is so lucky that we are able to weep.
It's a normal physiological manifestation to get watery when you yawn, and it indicates your brain is too tried. The secretion of tears happened all the time, and before you blink of an instant, tears left to the na so-lacrymal duct, you can't find treas are flowing. But when you yawn, you will open your mouth and breathe out carbon dioxide, gaseous fluid in your mouth can stop the na so-lacrymal duct excreting tears, so more and more tears gather together in eyes, and finally they welled from your eyes.
It is normal for every one.
The drainage system of your tears is called punctum. You have four puntums for each of your four eyelids which connect your tear gland to your nasal cavity.Tears often are produced to lubricate your eyes and will be drained out by punctums after that.
When a normal person yawns, his glands and punctums are stretched. The closing of punctum causes your tears to overflow and run down your face.
In other words, those tears already exist before you yawn, yawning is like squeezing those tears out of your punctums. Nothing to worry about, you can't be more normal.