A 55-year-old male with a history of right shoulder osteoarthritis underwent a total shoulder arthroplasty 7 months ago. The patient now complains of right shoulder pain, instability, and weakness. He denies any falls or other trauma since surgery. Physical exam is notable for weakness with the belly-press test and external rotation of the right shoulder to 110 degrees compared to 80 on the contralateral side. His radiograph is shown in Figure A. What is the likely cause of this patient's symptoms?