When a player declines after their rookie season, it is called a sophomore slump, or the sophomore jinx. There is a popularly held belief that after a successful initial season, a player in his second year will be jinxed and not have the same success. Most players suffer the "sophomore jinx" as scouting reports on the former rookie are now available and his weaknesses are known around the league.
A sophomore slump is far from universal. As pitchers tend to fare worse when exposed to the same hitters multiple times, a sophomore slump is not unusual. Additionally, a hitter may get regular playing time when they are on a hot roll and the law of averages would indicate that they would decline with increased time - for instance, Chris Duffy hit much better in a limited sample with the 2005 Pirates than his minor league track record would indicate. This earned him a regular role but in 2006, he declined as would be expected for someone who had been playing over his head. People judging him on 2005 alone and ignoring his minor league statistics could then call this a sophomore slump, but it's more a case of Duffy having been a "flash in the pan".
Some players manage to bounce back from a sophomore slump: Gary Carter was second in the voting for the 1975 National League Rookie of the Year Award, then hit only .219 in 1976, before bouncing back to have what turned out to be a Hall of Fame career.
- Michael Clair: "Rookie greats who never reached that level again", mlb.com, June 22, 2020.