Every team knows the Bills would be looking hard at CBs and you have to assume some teams would be willing to jump in front of Buffalo to grab the last CB with a 1st rd grade.
Bills were calling teams starting at pick #20 from what i understand, before they found their partner at #23. If they sat back and watch another team trade with Dallas they would of probably regretted it. So they made the move to get a guy they had ranked as a 1st rd talent at a position where teams were gobbling up the position.
It would be nice to still have the 4th, but not sure it would of been worth it as our need for a solid CB#2 was well known need.