Well, this is only my analysis. The main reason would be Laporta's dictatorship. I'm sure everyone noticed that since Laporta's take over, almost all high ranking officials of the club has a falling out with him. There must be in Laporta's leadership style that is disagreeable. In a sense, there is some kind of mismanagement going on. Barca's decline started in 2006-07, by then almost half of the original Laporta's men deserted him, also Henk Ten Cate left the Club. I think Ten Cate was a great coaching staff too. Then last season, Laporta almost lost the vote of confidence by a slim margin. There is definitely a dark side to Mr. Joan Laporta and I am sure it will be unraveled soon once he stepped down from power in 2010.
IMO, Ronnie's declining relationship with the Club eminated from above-mentioned premise. Plus the fact that his contract is due to expire and re-negotiation is needed. Most probably some terms in the agreement is not satisfactory to both party anymore. Image rights are always the one thing being disputed upon as this clearly applies to Ronnie.
Another thing is, Laporta attempted to sell Ronnie to Milan before the start of the 2007-08 season. Remember all sorts of news coming out that Milan is bidding for Ronnie. This was confirmed by Assis (Ronnie's brother/agent). I can recall reading Assis' interview " I don't know what happened in the negotiation of Barca with Milan, why it didn't push true." Which leads me to believe that Laporta intended to sell Ronnie without even first consulting Ronnie. I don't know, maybe I am wrong. Ronnie maybe felt unwanted so his desire to perform was diminished. There is no more goodwill and incentive to perform well because the player is not being appreciated anymore.
His moving to Milan is the time to re-discover himself again and re-focused. Anyway, time will tell. Ronnie has a 3-yr contract with Milan and after that who knows what is going to happen.