I really think it's because Belichick is doing a majority switch to 4-3, knows he isn't paying Adalius next year, and wants to get value back right now (similar to the rationale for the Seymour trade).
We play lots of nickel and dime this year, with our plentiful DB's, an offense which should give us the lead usually, and at linebacker play Mayo and Guyton.
Other than that, the D is fine with the 4-3 using Mayo, Guyton, and Burgess (who played a ton last game).
Like with Seymour, the value isn't there to pay a guy you're not going to put on the field a lot, thus a trade for future value.
