I was being glib about how many corners, and it's been years and years since I read the books. That said, what I've heard is that he doesn't fully outline the story first, and follows it where his imagination leads him (to a certain extent. he's obviously a successful novelist). But it has meant that there are a LOT of threads left to trim or weave together and only two (theoretically) books left to do it. Dany is still stuck in the east with a lot of important shit to do, the Dorne stuff and the fake Targaryen have to find some reason to be so prominent, the white walkers were handled a bit haphazardly in the show and show little sign of being integrated into the books overarching plot YET. He still has to decide how (and "if" I suppose) he wants to bring Jon back, whether R+L=J is still the way he wants to go with that, to say nothing of how he will actually end the thing... There are just a lot of spinning plates, and I can imagine he doesn't want to let any drop but may have to if he actually wants to finish this story in no more than 3000 pages (and even that will be a lot)