I bet that was cap related. Vrabel is getting up there and didn't play his best football last year. Think it's a really good pickup for KC, especially since they're switching to a 3-4.
Edit: Belicheck spoke about this today. Here's the rotoworld blub.
Bill Belichick, speaking on WEEI in Boston, said he never received an offer for a first- and third-round pick for Matt Cassel.
"They never made that offer to me," Belichick said, indicating there were no official late offers in the Cassel dealing. Without getting too specific, Belichick said he was in contact with multiple teams after Cassel signed his franchise tender and he took the best offer he got. He also indicated that the complexities of a three-way trade (presumably including Jay Cutler) could have taken a long time to play out, largely because it would have been contingent on new contracts. Reading between the lines, it seemed like the Patriots didn't have a firm offer or the patience to wait for one because they needed cap relief.