The reasons are many:
1. Cable subscriptions, dish sign ups, etc. are down across the country... more people are cutting the cable. So that will have an effect on ratings.
2. Someone mentioned inconsistent officiating. I think it goes beyond that, the officiating is so bad that it seems to be a rigged system. I don't know if it is, I'm just saying it seems that way. When you have an official that gives the ball to Team A after a fumble, when a player from Team B is standing 5 yards away with the ball, it's either complete incompetence or the official was told to do that. Either is unacceptable. And then the announcers, rather than saying things like that was a bad call (they do once it a while) they are getting very creative in justifying the poor calls. Like in the Colts game when TY's TD got called back for illegal shift and Solomon is trying to explain that it was because Gore's shoulders were not completely square the LOS.
3. A combination of, players showing disrespect for their country and for those that have died for our freedoms, even with that, I think fans would be accepting of that even if most disagree, but add to that the networks focusing so much attention to it and putting those players on a pedestal and acting like they are some great humanitarians.
4. Players want to celebrate everything... a guy gets a first down and he's flexing and jumping up and down, making the first down signal. It's a freakin' first down... congratulations you did your job on that one play.
5. IMO there is just too much football. Thursday night games, all day Sunday, Sunday night, Monday night. Early Sunday games when a team goes to London. It's not longer an event.
It is a problem the NFL is going to have to address. If the ratings continue to drop it will affect the contracts they receive from the TV stations which will affect the salary cap. What will teams do when the salary cap stays the same or goes down from the year before? They will be scrambling and then the problem will become worse.. because teams will dump the higher salaries on their teams which means more younger players which means worse play and more antics.