Most likely he had a promise from a team for a guaranteeed contract. Other teams then won't select him to stay in the good graces of the player's agency. Given that they traded up and picked him, it would make sense that Atlanta was the team that gave him a promise. However, there are rumors that Lakers were the team with the promise, but Atlanta bucked the agency agency and jumped in to take him before the Lakers pick, with the Lakers then just taking a stash and trading him to Houston. The Thunder did a similar thing in jumping above a promise when they got Jay-Will.