This stretches the eyelid and may cut off the drainage system. This is the most common cause and there is little we can do to correct it. Some animals are born with an abnormal drainage system that may or may not be surgically correctable. Sometimes the eyelids turn inward and block the drainage. This is also surgically correctable.
Hair can act like a wick, drawing the tears out of the eye.
This can be corrected by removing the offending hair. In some cases tear overflow may be due to excessive tear formation caused by irritation of the eye by a particle of something in the eye, an allergy, or an abnormal eyelid or eyelash which turns inward and rubs against the surface of the eye. These conditions, which are often painful, need to be seen immediately by your veterinarian. Since the tear accumulation on the facial hair can also lead to matting of the hair, skin irritation and possible infection, you must keep this area clean. Trimming the hair below the eye will help.