In the end, coach Bill Belichick must have felt Mankins' play and salary ($6.25 million)/cap charge ($10.5 million) were no longer aligned. Mankins may no longer have been elite -- his fourth-down missed block in the AFC title game comes to mind -- but he was certainly still playing at a high level.
So it appears his performance relative to cost this year and in future years ($6.75 million base salary in 2015 and 2016), the acquisition of Wright and the fourth-round pick, and also, the feeling that there are some capable young linemen (e.g. Josh Kline) in the pipeline created the perfect storm to spark a few-saw-it-coming trade. It was just a week ago, when asked about the offensive line, that Belichick said, "I think it's a real competitive group. They all work hard. They all have shown up well from time to time. We'll probably have some tough decisions there."
No one saw Mankins as being part of those tough decisions, especially given his standing as a team leader. As Buccaneers general manager Jason Licht, the former Patriots director of pro personnel, said Tuesday, "[I'm] a firsthand witness of what he means to the locker room as well."
The locker room void is significant, but Belichick, as we've seen over the years, takes emotion out of the equation to do what he feels is "best for the team." The Richard Seymour trade to Oakland in 2009 comes to mind along those lines, and there were some initial rocky moments in the locker room after that one.
By moving on from Mankins, it passes the torch a bit earlier than expected, as the team frees up some space to focus on some of their next contractual priorities, such as safety Devin McCourty, cornerback Darrelle Revis and offensive tackle Nate Solder.