Ruptured Atherosclerotic Abdominal Aortic Aneurysm

A 72-year-old male heavy smoker with hypertension and known 6.2 cm infrarenal AAA on surveillance presents with sudden severe tearing back pain radiating to flanks, syncope, and hypotension (BP 78/40 mmHg). FAST confirms free fluid. The autopsy specimen shows the structure marked **B** — a 7 cm fusiform infrarenal aortic dilation with laminated mural thrombus and posterior wall rupture into the retroperitoneum. Which of the following best explains why this patient's aneurysm progressed to rupture despite being under surveillance?