My reasons were simple. I've not finished last year's story yet, and I want to, and I've always planned for this year's book to be book three of my Black River cycle. So it made no sense to start #3 without finishing #2. Well, it makes some sense, but not to the extent this would be. And I have a bit of potential craziness involving Trisk in December that makes me want to shy away from crazy amounts of writing in November. I'd rather focus on one project and not t'other.
And then today, after a few hours of sleep, my brain was flooded with thoughts of a book I wrote a chapter of in 1995, and then never got back to. Yeah, I've been doing THAT little trick for awhile. There weren't any major NEW ideas floating around, but everything just seemed so clear, and while I've often dipped back into that well, poked around that world, and it's never been far from my thoughts, today it was just...wow. Just crystalising in my thoughts, and part of me really wants to get back into that world, big time.
Sooo. I have no idea what November will bring. Maybe I'll still say screw NaNoWriMo and work on these in my own time, although the entirely arbitrary deadline of that project always seems to help me get motivated.