What was the reason that Tony Stark designed the Hulkbuster armor in Avengers: Age of Ultron?
Was it made specially for controlling the Hulk, or was there a some other specific reason behind it?
Tony Stark worked with Bruce Banner himself on the Hulkbuster armor (affectionately known as Veronica) in order to stop the Hulk if he ever got out of control.
You have to remember that Bruce Banner neither trusts nor likes The Hulk as he doesn't really have much, if any, control over the Hulk after he has 'hulked out'. When the Helicarrier is attacked by Loki in The Avengers, we see that after transforming into the Hulk, Hulk attacks friends and foe alike, wrecking a large portion of the Helicarrier, hurting allies and eventually smashing a building to rubble after he is drawn away from the Helicarrier.
Although the Hulk is much more focused in the Battle of New York at the end of The Avengers and in the assault on the Sokovian castle at the start of Avengers: Age of Ultron, Bruce will always be wary of the fact that he is not in control after 'hulking out', and is painfully aware of the death, destruction and devastation that the Hulk has caused in the past.
"The last time I was in New York I kind of broke... Harlem"
Naturally, Bruce wants a contingency plan in case the Hulk ever goes off on a rampage again. That plan takes the form of a new Iron Man suit designed specifically to be able to fight the Hulk - a good job too, given the events in Johannesburg in Avengers: Age of Ultron.