Welcome to Android Central! It could be abnormally fast wear-and-tear of a physical button, or a defective button. Or maybe some residue of something seeped in and is causing it to stick. If it still works, there isn't much to do (and it's doubtful Samsung would agree to replace the button, since it's still functional). If it stops working, then you'd have to fix it. This is one of the unfortunate potential problems of a physical button.