When good hitters go bad sometimes they never return to their earlier standards. Baseball history is littered with one or two-season wonders or careers that suddenly derail in their late 20’s or early 30’s. And, of course, you can apply this to pitchers.
Sometimes it is injuries that take their toll on players. Sometimes it is just performance related where suddenly the wheels come off. It could be a Daniel Bard not remembering where the plate was located. And there is also the psychological component. Players often have a fragile psyche and poor can mentally migrate into awful.