My Sister Had Bleeding In Her Stomach From A Leaking

my sister had bleeding in her stomach from a leaking blood vessel this was found Dr. sent her to
emergency due to her low hemoglobin They went in and cauterized the blood vessel. she has
A-FIB and also she was told that she has stage three kidney disease
Can you tell me what would cause the vessel to leak. her diet has been normal Dr. has not changed it