A 52-year-old man from Mumbai with a long-standing history of hypertension presents with chest pain and dyspnea. On examination, blood pressure is 160/100 mmHg, heart rate 92 bpm. Cardiac auscultation reveals a late systolic murmur at the apex that increases with standing and Valsalva maneuver. Echocardiography shows left ventricular hypertrophy and systolic anterior motion (SAM) of the mitral valve. What is the primary physiological mechanism responsible for the dynamic nature of this murmur?