The Packers defense had some issues last year, so going defense makes sense with this pick. Still is a solid value here and with B.J. Raji in the fold likely projects to a defensive end in the Packers 3-4 defense.
SB Nation's Mocking The Draft explains the pick:
The Packers are one of those teams that can truly go best player available. With that in mind, it was fair and away Still, who also happens to be a good fit for their 3-4 front. There was no defensive tackle in the nation that played better than Still in 2011. He's a powerful player, but has the arm length and enough athleticism to line up at end.