Lacrimal fluid or tears produced by tear ducts flow either when a person experiences a very strong emotion or as a natural process to clean or lubricate eyes. Tears are secreted through two openings, the eyes and the nostrils. This happens because tear ducts and the nasal passage are connected.
Now when a person gets hit on the nose, the impact builds hefty amounts of fluids in the nose. This blocks the nasal passage and leaves no space for the lacrimal fluid to pass through the nose. Hence eyes begin to water.