I don't know. It doesn't really make sense to me, to be honest. I'm not proposing it was done to pump up EJ or anything like that. I could see the reasoning to cut Cassel, and the reason to re-sign him. To trade him for a 2017 late round pick sorta goes against the reasons to re-sign him, IMO.
He hasn't played for the Pats in 6 years and Rex has played them 12 times since, so I don't know what he could really bring to the table in that regard. I agree they thought a vet presence would help on game days, so why move him?