Roy Hibbert gets knocked for his lack of aggressiveness. Critics call him the soft interior of team with a hard exterior. He's shown he can be a pacifist towards "little dudes" like Steph Curry, but Hibbert showed in the first half that he can be downright nasty when the situation calls for it. Hibbert leapt over Johnson like he was standing between him and a rematch with David Lee. If you know Johnson's history with "anger management," you know that you can't be meek if you're going to try to humiliate him on national television.
Hibbert is 7-3 and Johnson didn't contest the slam because he appeared to be attempting to take a charge. However, he made two mistakes. First, half of his foot was inside of the restricted circle, which rendered his sacrifice useless. Secondly, he probably didn't realize Hibbert was going to be attacking the rim.
H/T to @SBNationGIF for the GIF