That was a perfect pass for that play. When the receiver is covered, the best way to complete a deep pass is to underthrow it. The receiver has a much better chance to come back to the ball than the defender. Offenses work on this so QBs and WRs are on the same page. Its no coincidence that you see that type of completion all the time, its designed into the play if the receiver is covered and the CB has his back to the QB when he throws. The CB not knowing the ball is coming cannot adjust, he has to play the WR and when the WR stops and goes back for the ball, the CB take a few steps forward before he can react, thus giving the WR a huge advantage.