Well, Palpatine must have put Anakin through some kind of mental-hell in order to physche him out of being a crybaby and suddenly becomming an obedient pet. Leia mentioned Tarkin had Vader at the end of a chain, not even Obi-Wan could steer Anakin and now a Moff has control of him - that tells you that in Episode 3 or afterwards, Palpatine smacks Anakin around a bit and shows him whose boss.