A 58-year-old man with a 20-year history of smoking and hypertension undergoes coronary angiography for stable angina. Angiography shows a 40% stenosis in the left anterior descending artery, but intravascular ultrasound (IVUS) reveals a large lipid pool with a thin fibrous cap. Which pathological feature of this lesion best explains why it carries a higher risk of acute myocardial infarction compared to a 80% stenosis with a thick fibrous cap?