The Duel of Johannesburg was an open confrontation between Iron Man and Hulk, who had been manipulated by Scarlet Witch.
Scarlet Witch, in conjunction with a plan she made with Ultron and her brother Quicksilver, used her powers to manipulate the Avengers, with the exception of Hawkeye.
Bruce Banner, affected by her powers, went into a state of uncontrollable rage and transformed into Hulk, travelling from the Salvage Yard to Johannesburg.
Black Widow would normally have calmed Hulk down, but she had been affected by Scarlet Witch's powers herself, preventing her from helping.
Seeing no other choice, Tony Stark activated the Hulkbuster armor to try to stop Hulk.