Skin contact is probably the #1 cause in my opinion. You can definitely see a huge difference over time no matter how they’ve been stored if one has been touched by human hands directly. I’ve always taken great care to make sure that coins I sell are never touched by human hands but you can never be totally certain what happens at the mint, etc. This is also a reason why I am opposed to coin grading. While I’m sure the grading experts know that touching coins with bare hands is a no-no, you can never be too sure that someone didn’t have a bad day or a moment of laziness while handling your coins.