A 54-year-old man with a history of inflammatory bowel disease (IBD) has a 3-stage total proctocolectomy with ileal pouch-anal anastomosis (IPAA) 9 years prior. He notes that there was a long delay between his second and third stage surgery. He has had significant diarrhea, fecal incontinence, and pelvic pain for years despite multiple medical therapies, including ciprofloxacin, steroid enemas, and rifaximin. He says he has never had good pouch function. What is the most likely cause of his current clinical condition?