I'll bite here. More and more are cutting the cable cord, which will hurt ratings on Monday and Thursdays (as those games are on cable). As for Sundays, the people I know who have cut the cord don't watch anything live most of the time. Thus that leads to the ratings drop on Sundays.
There's an argument that the traditional Nielsen ratings need some sort of update. Too many people are watching on alternative devices rather than TV's. It started with DVR's, but has only gotten progressively worse since the advent of "second-screens" and other ways to stream (Netflix, Hulu, Amazon Prime, Channel-Specific Apps, etc).
It's important to note that even though the NFL's ratings are dropping, they are still head and shoulders above all other pro sports leagues.