- Martin simply lost the plot and is unable to handle all of his characters and stories.
- The seed of Martin's nihilistic amorality finally blossomed into full flower.
- Martin made a foolish decision to change his approach to the story in mid-series and the structural changes that resulted have proven more than he can handle.
What about the idea that Martin's nihilistic amorality, always apparent, finally overtook the story, as per (2)? There is some evidence of this. The nihilism and amorality has certainly continued to increase as Martin kills off well-meaning Starks and replaces them with vicious new characters. One reader claimed that the theory can't be true because it was the nihilistic amorality that made the first three books appealing in the first place. Even if that is what set them apart, (and there is an element of truth to that; no one will forget the scene where Jaime chucks Bran out the window... for love), that appeal can't possibly explain the decline in the latter two books because Martin has undeniably cranked up the depravity in comparison with the preceding three.
The real problem with this idea is that we know all of this pointless viciousness is just a sideshow, filler for the events of the real story. While it might explain why the filler is so nasty, it doesn't explain why it exists in the first place. And that brings us to theory (3).
As most Martin readers know, the books were originally supposed to be a trilogy. Then, after the first book turned into three, Martin faced a five-year gap, during which time the dragons would grow and Daenerys would gather the forces necessary to challenge for the throne of Westeros. (One would think a wedding between Daenerys and Joffrey would have been the obvious solution to the situation inspired by the historical example of Stephen and Maude, but never mind that.) Instead of simply writing 10 pages of "here's what happened in the intervening years", Martin went back to write what was intended as one book, then became two, and has thus created an even more difficult challenge for himself by adding all of the new characters, many of whose stories are of little to no interest to the fans of the first three books.
Instead of proceeding with the 9 living perspective characters from A Storm of Swords, Martin now finds himself saddled with 21. This is a structural challenge, and if Martin somehow surmounts it to produce books that are on par with the first three, it will be an epic literary feat indeed. But the very fact that Martin faces it tends to indicate that he is not up to the challenge of defeating it.
Unlike many Martin readers, I very much doubt that Winds and Dream will be as bad as Feast and Dragons. After all, Martin is now returning to the story that he originally intended to tell, so he probably has a much better idea of what he wants to do. However, the unnecessary structural challenge created by all the additional characters and their stories likely means the last two - or three - books will not be as good as they would have been had he not given in to the temptation to go back and fill in the blanks of the required five-year gap. And that is an excellent object lesson for all writers: don't give in to prequelitis. Look forward, not backward. And don't hesitate to leave some historical blanks unfilled; indulge the reader's imagination rather than attempt to leave no stone unturned in explaining everything about everyone.
As for the theories of collapse, they may all be true and help explain each other. The unfortunate decision to go back and fill in the blanks led to a long submersion into the depths of pointless nihilism, which ultimately proved as uninteresting to Martin as it did to his readers.