Originally Posted by unnoted
Its Naruto's will i think. As Naruto wants to do simething impossible like beating Oro, he go on outrage and cant control himself to go on Kyuubi mode. he did controlled it once during the Sasuke confrontation.
Something like that, initially the Kyuubi's power was triggered by heightened emotions and when in dire danger at first only a bit would slip through like against Haku but after Ero-sennin's training he had better control and could call on it more. So yeah Naruto at the bridge he gets emotional because he's thinking he can finally get Sasuke back but he knows he's not strong enough to take on Oro so he calls on more and more of the Kyuubi power and when Kabuto and Oro keep taunting him he gets more and more pissed hence loosing control to Kyuubi and going into 4 tailed state.
All the states before the 4 tails he can control because his own chakra is strong enough to keep Kyuubi's in check but after the fourth tail it's too much and he goes psycho.
btw you should have spoiler tagged this or at least put it in the manga section there could be some spoiled anime only watchers.
and this isn't a newbie question you hardly understand anything in the manga reading it first time through
Hope I helped.