This can be due simply to aging, but also genetics, maybe a previously undetected cardiomyopathy or a viral infection.

As we age, our hearts become more disordered. It's natural. Some of us develop arrhythmias of a kind, or we react poorly to a vaccine, or a virus infects the myocardium and the heart begins to deteriorate. Some of us begin to deposit collagen in quantities in the interstitial tissue, and we get what is called fibrosis. These developments can make the heart less efficient, so it begins to grow more muscle in an attempt to make itself more responsive to the demands your body and your habits place upon it. This combination of thickening does the opposite, and your ejection fraction will begin to suffer, meaning your large ventricles will not fill with as much blood upon relaxing in each stroke as they did 20 years ago. Less blood in each ejection means less capacity to run the body.