Intraoperative tissue injury. Major causes and preventive measures

The patient has been on the OR bed for seven hours undergoing an abdominal aortic aneurysm repair. During the postoperative assessment, the perioperative nurse notices a reddened spot over the patient's sacral area. Why did this injury occur? Was it caused by the length of the surgery, hyperthermia blanket, pooling of povidone-iodine, decreased circulation, or another factor that occurred during the surgical procedure?