You're thinking about failing, and getting ready to fail. You're scared of the pitcher, scared of the ball. You get revved up. You forget what your coaches say and swing crazy, hoping to get lucky. Or you stand like a statue while the umpire calls a strike.
Most young hitters give up now.
They swing at the next two just to get it over. They don't grow in the at-bat. The bat's a white flag, and they're waving it to surrender. To have a good rookie pitch, you have to be good inside. Good rookies go up to the plate respecting the pitcher, and humble about their odds.