A 72-year-old woman from Bangalore with a history of atrial fibrillation (not on anticoagulation) presents with acute right-sided weakness and facial droop of 6 hours duration. Non-contrast CT head is normal. MRI is performed: DWI shows multiple acute infarcts in the left middle cerebral artery (MCA) territory with restricted diffusion. ADC map shows low signal in the same regions. What is the most likely mechanism of stroke in this patient?