Bunch of different reasons they might do this. Kind of odd, though, but unless it's happening every year or something, would just say the team decided after drafting him they made a mistake and decided to trade him for what they think they could get. Maybe after drafting him the head scout went to the owner and said, hey, I made a mistake. It was Polermo Malfosia, not this guy who was good! Better trade him while we can.
Hopefully in future versions the trading of drafted players will be more like it is now. Think you have to wait a year before you can trade them? Could be wrong...