A 35-year-old zookeeper fell 10 feet while preparing an exhibit for a grand reopening, landing on his left arm. The patient is then evaluated by a keen orthopedic resident in the emergency room who describes the zookeeper's injuries to his chief. He describes a comminuted radial head fracture and posterolateral ulnohumeral dislocation. The chief resident orders a CT scan which demonstrates a coronoid fracture involving 50% the height with no involvement of the anteromedial facet. During surgery, the trauma surgeon replaces the radial head and repairs the lateral collateral ligament complex. The elbow is splinted in elbow flexion and pronation. The patient begins range of motion exercises with her occupational therapist 3 days after surgery, and her elbow dislocates. What is the most likely reason for her instability?