One of the enormous moments (of the many) in which audiences cheered wildly was the revelation of Bruce Banner turning from his friends, stating simply, "I'm always angry," and single-handedly taking down a Leviathan with a solitary punch after his transformation into the Hulk. It's also a moment that a lot of people consider a "plot hole" (which isn't even the correct term seeing that it doesn't create a fallacy in the narrative of the film), because Banner apparently goes back and forth in the movie of not being able to control the Hulk to conjuring him up whenever he feels like it. Allow me to explain. You can take one route, the route of the ending of The Incredible Hulk which ends with Banner meditating and seemingly choosing to let the Hulk out in the last frame of the movie. That should set up the whole "can control it at will" device. Another is that Banner has to be completely conscious to control the Hulk and that, in times of great duress and pain, the Hulk does just come out, if anything, to protect Banner. In the scene in which Banner and the Black Widow are thrown into the lower levels of the Helicarrier, Banner has suffered a head injury and begins to lose control. This, complimented by the mind-control of Loki's staff, puts Banner in a state where he can't control the Hulkish outburst because he has no means of focus or restraint. Remember in the movie when Bruce admits that he's attempted suicide, but couldn't commit to it because every time he tried, the Hulk appeared to stop him? That's emotional duress, keeping him from focus, which makes it easier for a Hulk cameo. Bruce can control the Hulk when he's level-headed. But when he's under the influence of an outside force, he cannot. Simple stuff, really.