If I had to guess, it's the mitral valve. When they replace one outright, there has to be disrupted tissue, maybe tissue that is incapable of repairing itself fully and restoring the original surface geometries involved. When blood flow is turbulated by disturbed/altered tissue surfaces, it might increase the probability of clotting. Again, just my mind working on what I know from other instances of a similar kind. If your risk of clotting arises, maybe from a history of AF, even if remediated, and your left atrial appendage was not closed off (LAA), and you have had surgery to repair a valve, your risk is generally higher for clotting and eventual stroke. If this is what the reasoning is, then it follows that mitigation of risk can be achieved by asking the patient to use the prophylaxis afforded by a DOAC (direct-acting oral anti-coagulant).
But there may be other factors that the CHA2DS2-VASc calculation takes into measure, and maybe it's just that score that your cardiologist is going by. If your score is 2.0 and above, it would almost certainly require the cardiologist to ask you to take a DOAC.