Brock was getting booed before he ever set foot in the Octagon. Fighters that he has gone on to destroy (Mir/Herring) have completely dissed his pas career and show zero respect for him.
It's baffling how much hate Lesnar has gotten. I attribute most of it to bitter Fedor fanboys, but you're right, he was hated from the start. Jealousy I'm guessing.