"One line of dialog, I believe, dealt with that. "I'm always angry."

Banner is in denial about the Hulk part of his personality when we first find him. He assumes that as long as he's in control, as long as he's not angry, the Hulk can't come out. Only that's not how the human mind works. You can keep yourself calm, you can manage how you respond, but you can't stop yourself from feeling angry. It's not like Banner wasn't angry for the last year. He was in denial, and that denial was preventing him access to another part of who he was. When the Hulk came out previously, it was under the mistaken impression that there were two parts separated by anger, but when Banner realized he's always angry, the two parts separated by an imagined wall became one.

Once he realized that he was always angry, the illusion that anger separated him from the denial of him being one and the same with the Hulk came crashing down and Banner/Hulk emerged as one person."

By an io9 poster, who got it right, I suppose.