I really like Watt a lot, but was happier with the trade down as I don't see him as being a tier above several of the other players still on the board (Bowser, Willis, King, Awuzie, Lamp especially). He's probably my first choice out of that cluster, but I'd much rather have my pick of the rest plus a 4th rounder.
But I'm not surprised to hear that they had a 2nd round grade on him. Most of the reports you hear indicate that teams average about 20 first round grades per year. Players with 2nd round grades go in the 1st round routinely just out of necessity. So the fact that they had a rd 2 grade on him doesn't at all mean that he wouldn't have been the pick if they hadn't been able to find a trade out.
My unsubstantiated theory is that once Tre White was taken at #27, they decided to pull the trigger and move down. I'm guessing he was their last 1st round grade left on their board and they were waiting to see if he would make it to their pick.