I think it has to do with this:
Young player proves himself in debut. -> pressure is taken off and public becomes forgiving if he makes mistakes. Then in 2nd season young player feels pressure to develop his game and encounters trouble doing so.
True. This is likely the reason top class teams are very patient with their kids. It's tough to handle not your initial season when it's pure adrenaline, but subsequent followings.
At Dortmund you get 3-5 years of low pressure. Ok, make that 10, but anyway, you get the picture. At Barca, after one season, they give you number 10, new Messi, new Iniesta status, and the pressure kicks in big time.