It does exist, but not necessarily due to a dip in form, but from the opposition sides becoming aware of the new player and his strengths and weaknesses.
In the first year, the new player is a bit of an unknown quantity with the other teams in the league, so no one knows how to handle their threat initially. In the second year, the other teams have learned about the player's abilities and know what to do to contain the threat, plus they may have spotted weaknesses in the individual player's game and might focus on exploiting them as well.
For example, Craig Calvert came in to the squad with lightning pace and lots of tries, but when the other teams knew about him they were closing the gaps that allowed him to score. In the last season, a confidence issue had become apparent and most teams focused their kicking game to his side of the field knowing they could force an error. He's still a good player and can get us tries where other players can't, but he has a few areas of his game to work on if he's to serve as an asset rather than a liability in the big games.
That's my understanding of "second year syndrome" anyway.
