Stark had been saying all along that Banner needed to let go - i.e. to let himself change into the Hulk for a purpose (at least, that's the implication I took, and I think that was pretty strongly implied.) When he changed first, he had just been arguing with the others and was already angry with them. He was distrustful and felt a but betrayed or used. All this was exaggerated by Loki's influence (at least, again, I think that was the implication of the scene with the staff). So when he was unexpectedly hurt from the fall and unwillingly transformed, it was just berserk Hulk, attacking those he had just been angry with as Banner.
Everything about the context of his transformation at the end was different.