GM R.C. Buford at 2:24 mark in the video points out that the Spurs draft was built around getting Joseph with 29th pick.
This allowed the Spurs to trade beloved guard George Hill to his hometown Pacers for the 15th overall pick. If Spurs told Joseph of their plan in advance, this would help explain why Joesph was willing to keep his name in the draft. Give Alexis Wangmene the rare assist.