Massive gastrointestinal bleeding after abdominal surgery, other causes besides surgeon error?

Ulcer, blood thinner. Gi bleeding after surgery can be due to stomach ulcers which can arise from stress. Also, many doctors use blood thinners to reduce clots in legs. These blood thinners can cause any fresh wound to bleed. Definitely, ask your doctor.
Many. There are many causes of gastrointestinal bleeding that are not related to surgery. It can depend on if the person is on medications that prevent blood clotting. Bleeding can come from many sites in the GI tract or abdomen, away from the surgical site. It also depends on what "abdominal surgery" was performed, if the surgery was for an emergency problem, patient's past medical and surgical history.
Many. There are many causes of gastrointestinal bleeding that are not related to surgery. It can depend on if the person is on medications that prevent blood clotting. Bleeding can come from many sites in the GI tract or abdomen, away from the surgical site. It also depends on what "abdominal surgery" was performed, if the surgery was for an emergency problem, patient's past medical and surgical history.