your eyes water because your ear, nose and throat are connected and when you get a cold your frontal sinuses(which are located in the middle of your forehead just above your eyes) are put under pressure, your eyes watering are caused by the irration and pressure from your frontal sinuses and your nose
I think it happens because the pressure on your sinus. Tears drain from either your lacrimal ducts or tear ducts and they run between your ears and nose so any pressure on the nasal cavity can effect your tear ducts.