Possibly because they are just sitting and not being used. Not that all will go bad but non use is not always good either. I have some non use lithium too but have not checked in a long time. Im sure I'll run into that issue myself when I come to finally check them. Its kind of like someone pressing on your skin for years without letting up. Your skin breaks down and damage occurs. Im sure batteries are similar. Use them and they should do you well. I remember one person who I believe still recommends you not buy your batteries until right before you put them into use. I believe this would be good practice and one that many don't do.