AF shares different conditions to atherosclerosis (i.e. risk factors) and AF confers a worse prognosis in atherosclerotic disease, on the other hand, atherosclerosis confers a worse prognosis in AF patients.

Perhaps we could consider AF as a marker of vascular disease, and could stratify these atherosclerotic patients who develop AF as they are at high risk of vascular complications. This hypothesis is the same way when we consider left ventricular hypertrophy or proteinuria confer a worse prognosis in patients with hypertension