What Causes Bell's Palsy?

I have an aunt who has bell'spalsy for 2 times who is nowatthe age 66 yrs. old now.She a diabeticyear 2005. shes obese, at around 5''height. i observe her right eyes usually tears coming out and she was diagnose that she has a cataract and now was operated but it also tears comes out. I want to ask what causes bells palsy