There's definately an argument that the calls are slanted to the offense when it comes to the passing game. Over the years the league has made it so that DB's can't do much of anything without drawing a flag.
I also think with the defenders having the physical edge taken away from them through officiating, the type of defenses have changed a lot over the years. You see more of the "speed" defenses since they have to protect against the passing game (Indy for example). This would account for more craziness in the rushing game--especially rushing TDs.
imo, anyway.