We know that in this scene, The Hulk's rampage is triggered by Scarlet Witch messing with his brain, indicated by his somewhat reddish eyes. Over the course of the fight, he is clearly out of control and attacking anything or anyone that gets in his way - quite unlike how we see him act in The Avengers/Avengers Assemble, where he seems to be able to prioritise targets.
After a certain part of the fight between The Hulk and the Hulkbuster suit, we see that The Hulk seems to be dazed, or somewhat confused - we see a shot or two of him seemingly "coming too", his face showing what could be concern or sadness about the destruction he has caused as he sees people running and buildings collapsing. At this stage in the fight, his eyes appear to have gone back to normal.
We have seen from The Avengers/Avengers Assemble that "cognitive re-calibration" (hitting someone really hard in the head) is enough to stop someone from being under mind control. It is my belief that the same thing happened here, at which point The Hulk began to revert back into Bruce Banner after one final punch from the Hulkbuster armour subdued him completely.
It is also worth mentioning that at one point in the fight scene we see the Hulkbuster suit seem to spray some kind of chemical on The Hulk, which may or may not have contributed to his defeat.
Additionally, I think it is safe to say that the movie version of The Hulk has more limited power than the one seen in the comics, based on what we have seen so far in The Incredible Hulk, The Avengers/Avengers Assemble and Avengers: Age of Ultron, most of which have him being subdued in ways which would not usually affect The Hulk in the comics.