For everyone of the mentioned players their own reasons and the circumstances that, most probably, led them to transfer to a rival club.
Crespo: He wanted to climb the ladder of the Italian football step by step: Parma then Lazio then Inter. After that, he was sold to Chelsea, despite the fact that he loves Italy and he didn't want to leave. Everything has turned into a mess when he transfered to Chelsea, he didn't adapt in Stamford bridge and he wanted to return to the peninsula. Milan was the one who approached him and he was loaned to them. Then he returned to Chelsea and the same thing happened, he was loaned to Inter eventually.
Pirlo: He wasn't playing regularly in Inter, he preferred to play for Milan. Then, Milan re-discovered him and he won all possible trophies there. On the contrary was his days at Giuseppe Meazza.
Baggio: Juventus was the one who sold the Divine Ponytail, however, he'd been Juventus and Juventus were him in the five years he spent in Turin.
Ronaldo: A fat traitor.
Inzaghi: His situation was nearly similar to that of Baggio's. Plus, he didn't accept being a third choice striker after Del Piero and Trezeguet. Hence, he preferred to leave. I can't blame him though.
As you can see, Trezeguet's situation at Juve has been better than the previously mentioned players. He's been playing regularly since his arrival, highly respected by his team-mates as well as the fans and he won lots of trophies with Juve. Even though, he perhaps asks to leave due to our demotion, seeking different experience, etc...