Injuries are the biggest reason for this, IMO. The Colts' injuries are well documented; the Titans lost Vince Young for the season, Kenny Britt for 6 weeks and maybe have one player (Jason Babin) who has been healthy the entire year on the defensive line and hasn't missed a game; and the Texans lost DeMeco Ryans for the season. Maybe a Jaguars fan can fill me in, but I don't know if they've suffered a huge injury like the other teams besides maybe David Garrard out for a game.
The teams are still immensely talented, but with the Colts' injuries, the Titans' quitting on their head coach and the Texans' inability to win close games and play pass defense unless they're playing Rusty Smith, the standings will reflect each team's flaws this season.
Aaron Kampan was having a great year for the Jaguars. He only had a few sacks but his QB pressures were a big help and freed up other pass rushers to get to the QB. We also lost Eben Britton who was our RT. He was doing a great job for us too but it looks like Jordan Black is filling in nicely for him.