Well, given Seymour's history with them, I'm sure the Pats didn't consider him a "team player". Translation: he had balls, and stood up to them, LOL.
He's almost 30. He's been slipping a bit. He's not as good in the 4-3. He's in his final year. They knew they probably weren't going to be able to keep both Wilfork and Seymour, and obviously valued Wilfork more. They're deep enough on the D-line to absorb his loss with the least impact. (Note: I'm not saying they won't miss him, or wouldn't be better with him. Of course they would. But the dropoff without him, considering the other talent on D, obviously wasn't worth his cost or losing the chance to get a 1st-rounder for him.)
The Pats are all about value. It would be nice to have A+ players at every position, with A- players backing them up. It's not realistic. Hell, most teams don't have ONE A+ player, let alone more than one, let alone backups better than a C+. On the D-line, they had a lot of A+ and A starting, and a lot of B+ backing up. That's a lot of cash. They must've figured they weren't getting value, having Seymour starting and having the backup guys ride the pine.
The problem with this trade is the injury unknown. It's like the Red Sox, every frickin' season. If I hear, in April, the words "the Sox are loaded with starting pitching" ONE MORE TIME I'm goin' postal. Every April we hear about how they have "six or seven MLB-quality starters!" And every August we're bemoaning the lack of starting pitching as it cripples their chances. Yeah, it looked good in April... until Smoltz and Penny washed out, Daisuke turned into a $50M albatross, the 78-year-old Wakefield got dinged up, and now Beckett has magically lost it. So, from where we now sit, it's easy to say th D-line is deep and can weather the Seymour-less storm... until the Bills game, if (God forbid) Green and Wright both go down with season-ending injuries, then suddenly we're screwed.
I still like the trade. They got a 1st--potentially a high 1st--for a guy who was probably walking after this season. A guy who's approaching 30 and whose best years are probably behind him. They drafted him 6th IIRC, got many Pro-Bowl years out of him, won three Super Bowls with his input, and traded him for what might be--a 6th. You can't ask for better than that. (Unless it's Randy Moss for a swindle, LOL.)
I would be willing to bet the Pats weren't LOOKING to move Seymour. But maybe the Raiders called with, to quote the great Don Corleone, an offer they couldn't refuse. Look, we love the players on the team, but no one's untouchable for the right price. Not even Brady, probably. Think about it: the way Cassel stepped in last season, with Brady coming off a bad injury... if someone had called Belichick in March and said, "We'll give you this years 1st and 2nd, next years's 1st and 2nd, and 2011's 1st and 2nd, for Tom Brady," Belichick WOULD entertain that deal. It's what he does best: putting the team first, putting them in position to be successful down the road, and making sure they're not a one-year wonder like the Bucs and Raiders in '02.
As the Pats players like to say, "It's a business."