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.