What causes a fighter to become "gun shy"?

We see it with some fighters who used to be a lot better of strikers start to become tentative on the feet. Unwilling to throw strikes when they normally would. Guys that come to mind are Rashad Evans, KJ Noons and Anthony Pettis even seems a bit like this lately. What causes it? Is it that there timing is off from not enough sparring? Or ring rust

Could. Add JDS to the list too