There were two corners taken in the three picks before him—M. J. Stewart and Donte Jackson—so I think BB saw a run coming. (Two more CBs were taken in the rest of the round—Isaiah Oliver and Carlton Davis.) And we know that he didn't have much interest in the 4th round last year (he traded another 4 to get DET's 3 this year). So I think he decided that trading away the 4 to make sure he got Dawson was worth it.