How did Jerry Johnson lose his hitting abilities

Jerry Johnson (Billy's favorite player) according to Billy seems to have been 1 of those slugger types that put up 30+ homers and 100+ rbis a year. We are led to believe that in the movie he is 35 or older and still playing but has been demoted to the bench. If this guy is supposed to be as good as he is, then what happened to make him have such problems hitting?
The movie does not mention any injury ever happening to him which causes Johnson not to be able to hit a ball good anymore. In batting practice or in a game whenever he swings, every single time he hits the ball right into the ground. How does a hitter suddenly lose the ability to hit the way he has in his career, especially when he's probably been doing it for at least 10 years or longer? During batting practice I'm guessing he was thrown some fastballs, but each one got pounded right into the ground. I've seen players lose their abilities with age, but every single pitch he hits goes into the ground? Sounds kinda fake to me, but I could be wrong. I can't recall ever seeing that happen to any other washed-up old hitter. They would at least hit some fly balls as well. Fake or real? Comments?