Laparoscopic left colectomy was performed for a 4-cm adenocarcinoma found at the mid-descending colon. A tension-free colorectal anastomosis was performed using the distal transverse colon at the splenic flexure as the proximal conduit. On postoperative day 4, an anastomotic leak was seen on computed tomography (CT). What is the most likely reason for failure of this anastomosis?