My muscle turned to fat.
Well, no, it didn't. I know, I know. You stopped lifting and suddenly you realized your strong legs and tight glutes and solid back have become a loose bottom and a beer belly. But it didn't "turn" to fat. It was replaced by fat.
This isn't a wholly crazy idea - water turns to ice, after all, and ice turns to water. So why doesn't flesh just jump from "muscle" to "fat" and vice-versa?
Because unlike water and ice, muscle and fat aren't just different chemical states of the same substance. They are different substances. Muscle is active tissue that can contract forcefully in order to move the body or act on external objects. Fat is an inactive tissue - it's basically just fuel storage. Fat in food - and fat in you - is the most calorically dense substance the body can use for fuel. So it stores it that way - it's the most efficient way to carry around some extra fuel for when you don't have enough coming in or have some extra demands.
Muscle is calorically demanding - it takes energy to run muscles, and they must keep getting energy to maintain them. Fat isn't demanding. It takes energy to keep it alive and healthy (blood flow, etc.) but not very much of it. Muscle does things, but costs a lot. Fat is your emergency savings, and it's not costly to hold on to.
Your body needs a reason to keep muscle around - demands on your strength, say, or endurance - but it needs a reason to get rid of fat.
So is that why former strong guys, like NFL players, get fat? Pretty much, yes. It's not that their muscle "turns" to fat, it's that their exercise load diminishes, so the body is less likely to hold onto muscle. Meanwhile the same calories come in, and get stored as fat.
The same thing happens to you. If you change your exercise load (i.e. stop working out) but the calories come in the same, your body starts to get rid of muscle by burning it as fuel ("Don't need this costly stuff around . . . ") and storing more fat ("Might need this later . . . ").
Muscle doesn't turn to fat, but fat does replace muscle. And vice-versa. So now you know - next time someone says their muscle turned to fat, remember it's just that they don't realize they are two different substances.