Why would a girl have an adhesion on her bowel and bladder?

Surgery/inflammation. Surgery and inflammation are the usual causes of bowel adhesions. While fewer adhesions form after minimal access (laparoscopic) surgery, some still do form. Inflammatory causes of adhesions include diverticulitis, pelvic inflammatory disease, inflammation of the gall bladder.
Peritoneal adhesions. Adhesions in the abdominal/pelvic/peritoneal cavity result from inflammation. This can be from surgery or injury, from infection, inflammatory bowel disease, chemical irritation (such as blood, )and from foreign material. You do not state age: in reproductive age females endometriosis, infection, surgery (especially cesarean delivery) are common causes. A clear cause is also not always identified.