It's the vocal cords and changes in the throat and other tissues. Plus the result of attempts to repair existing damage. Julie Andrews' career was destroyed by a botched operation to address some age/overuse issues with her vocal cords.
I'd be so devastated if I was a performer and the one thing that I needed to perform was failing/failed. They make money on their voice. Two strands of tissue that are moved and stretch and vibrate driven by their lungs. (I wonder how many singers have lost heir careers due to lung issues too)
Overuse, and injury. All athletes fear it, all people that use parts of their body fear it happening. It's unavoidable, sadly. unless people use technology to counter it. Like performers that are forgetting lyrics too. Jimmy Buffett has gotten better at staying to the lyrics of his songs, but he's using a small screen to help him. He's still a great performer, and I'd go to any of his shows. Parrotheads go for the scenery, and the parking lot. *shrug*