After reading Jack McCallum's book it was clear the attention the two other stars got rubbed Marion the wrong way. I don't really blame him for feeling that way.
Nash and Amare got all the credit and played zero defense. Shawn did all the dirty work and was a better rebounder than Amare. By all accounts he wasn't a cancer but that might've changed in the weeks leading up to the Shaq trade.
That was a great book, forgot about that altogether. I think Marion was fine being 3rd fiddle to Nash and Amare but he wasn't happy how he was always the "movable piece" as far as trade rumors and also it seemed JJ became prioritized over him, before the deal moving him was finalized which made Marion feel less important than he was.