A 58-year-old man presents to the emergency department with acute substernal chest pain radiating to the left arm, onset 6 hours ago. He has a history of hypertension and smoking. On examination, BP is 95/60 mmHg, HR 110/min, and JVP is elevated. Auscultation reveals a new holosystolic murmur at the apex. ECG shows ST elevation in leads II, III, and aVF. Echocardiography reveals severe mitral regurgitation with a flail posterior leaflet. What is the most likely pathological mechanism underlying the mitral regurgitation in this patient?