Manning: Not entirely sure why. My guess is that during the offseason, team defenses studied the heck outta him and that offense, and tried to find ways to beat him, or at least contain him (especially division opponents, since they have to play Indy twice during the regular season).
LT: Part of it I think is Rivers making some dumb decisions. LT I think has finally had it and is starting to show his frustration with the young QB. A few times today LT was open and Rivers ignored him. You have one of the most prolific backs EVER and you ignore him on little dumps. What the heck? Also, I think much like with Manning, team defenses watched a LOT more film and are figuring ways to better defend him and that offense. Plus, and what I think is one of the key factors, is their starting opponents have been brutal. Chicago, NE, and GB?! Wow...

Some are saying it's because he's "rusty" or whatever because he didn't play in any of the preseason games. I don't think so. He's done that since he was a rookie. He's used to that. He's smart, he knows the plays.
LJ: Some say it's because he now has a big contract and he doesn't have as much of a chip on his shoulder. Eh, possibly. I think it's more because their passing game is extremely WEAK. Their vaunted O-line has degraded over the years due to injuries/retirements, and now Tony G is having to stay back a lot more in helping defend. He can't help open things up and keep defenses honest like years before, which helped open up that running game. Also, another major factor is the starting defenses they've played against have been stout. Well, a much improved Houston defense, then Chicago, and now Minny? Wow... another brutal start for a top running back. :(
Not sure how all three will pan out by the end of the year... schedules the rest of the way are kinda up and down...