From his press conference announcing the deal.
Well, there you have it. Apparently one of Jackson or Tatum has been their guy all along. He also said this was a unanimous decision from the organization, including Brad. He also said this could lead into another deal, and they've already received calls regarding the 3rd pick, though they're happy with this trade regardless of any other moves.
Hard to argue the logic here. You might disagree with the draft analysis, but if they were going to take Jackson/Tatum anyways, then this is a no-brainer.
I'm guessing the bad Fultz workout sealed the deal for them. I still think it's Tatum.