A 55-year-old woman with chronic peripheral arterial disease presents with sudden onset of severe left foot pain, coldness, and cyanosis. She reports claudication for 2 years. On examination, the left foot is cold, mottled, with diminished sensation in the toes and weak ankle dorsiflexion. Femoral pulses are palpable bilaterally, but distal pulses are absent on the left. CT angiography shows occlusion of the left popliteal artery with no visible embolic source. What is the most likely mechanism of acute limb ischemia in this patient?