I have to admit I was pretty surprised to see this as well, since this award is so prestigious and I always am happy when Ayu wins. (Unlike some other awards that I really could care less about) But as others have said, I'm sure she has her reasons. She HAS won three times in a row now, and to me the idea of her stepping down because she actually doesn't want to win again doesn't seem all that strange. Another thing, though, that crossed my mind is that maybe she wants to step down because she sees all these other "big" artists stepping down as well, and sees the chance to come off being generous or something. I really wouldn't believe that myself, but it was a thought.
Oh well, it's not to say that she won't be participating next year!