Watery eyes during a bowel movement happen because straining increases pressure inside your body. This pressure can affect blood vessels and nerves around your eyes, leading to tearing.
When you push hard, you often hold your breath. This is called the Valsalva maneuver. It raises pressure in your chest and head, and that pressure can squeeze tiny blood vessels near your tear glands.
This is not the same as crying from emotion. It is a physical reaction to pressure, similar to what happens when you cough hard or lift something heavy.