A 1-year-old previously healthy baby boy presents to the emergency department with 3 hours of intermittent abdominal pain, vomiting, and one episode of dark red stools. On exam, his abdomen is tender to palpation, and there are decreased bowel sounds. A CT scan reveals air fluid levels and a cystic mass in the ileum. Gross specimen histology reveals gastric tissue. What is the cause of this patient's problems?