Realistically, he was able to be knocked out because there was no other way to end that scene. However, if we're looking for an explanation in-universe, I can see two possibilities, based on what little we know about The Hulk so far.
Not All That Angry
One option is that he was not really that angry with Tony at the time. Starting with the climax of Avengers, we've seen that Hulk is at least capable of distinguishing enemy targets from friendlies and civilians, and appears to be growing more coherent. It's possible that, even within his artificially induced rage, there was some portion of his mind that "knew" he should be directing that anger towards Scarlet Witch. He may have even recognized Tony as a nominal ally.
It's possible that this tiny bit of doubt was enough to prevent him from growing perpetually angry. He was Hulking out mostly out of instinct: he was sent into a rage so he smashed things. But he wasn't really angry at anyone. Perhaps that was enough of a subtle difference to keep his strength reigned in, enough that
Hulkbuster could knock him unconscious.
Not The Same Hulk
The other possibility is that you're just factually incorrect about his power.
It's well established by now that in the mainstream Marvel (-616) universe, Hulk is essentially unstoppable. The more of a beating he takes, the angrier he gets, and the stronger he gets, with no known limit.
It's not at all established that the Marvel Cinematic Hulk has the same potential. In fact, it would be silly to think they he does, because there would be no more dramatic tension. Hulk shows up to fight, everyone else loses, end of movie.
From his appearances in the three movies thus far, we know he is impervious to "injury". To my knowledge, we've never seem him wounded (bullet bounce off him, etc.), and he survived more than one fall from a plane to solid ground apparently unscathed. However, he does not appear to be completely immune to all damage from a brawl. During both his fight with the Hulkbuster armor, and his fight in The Incredible Hulk against Abomnation, he was clearly being worn down by the amount of a beating he was taking.
In particular, remember that
Banner himself helped build the Hulkbuster armor.
It seems unlikely that he would bother if he didn't think there was at least a possibility that it could contain him. Again, it's unlikely (perhaps impossible) for even the Hulkbuster to injure or kill The Hulk, but that doesn't mean it can't render him harmless long enough to shift back to Banner.