I hate when this happens – I am mere chapters away from The End in my current novel, slowing down a bit because I start to feel funny around this time, every time – and then it happens. There I am, in the bathroom, minding my own business and contemplating how this next chapter needs to start, when suddenly BAM!
I get this new idea for a tension-building twist.
Right out of left field, this one. And I admit, it would be cool. But there are logistical issues, can it happen logically or would this be a big stretch – one of those “Duex Ex Machina” nightmares?
I dunno. And that makes me nuts, becuase NOW I have to sit here and contemplate it all, see if it will fit, if it won’t fit – and oh Crap, I’ve just had another twisting version of the idea pop up.
Frak!
*wanders off to sit and THINKTHINKTHINK instead of write, dammit*
The Velvet Zebra 
Speaking as someone to whom this happens all the time, I always come off wondering if this is a defense mechanism built into our brains for some really dopey reason. We are preserving ourselves by preventing the ending from happening.
But mostly, what I think it is, is that you live inside a novel and inside a certain walled-off set of ideas for the length of a novel, working and building and thinking about it and turning it over. And you can’t help but generate new ideas. It’s that or just endlessly refine the old ones. And the brain quits on that pretty quick.
Finally, I sometimes think that it’s because we become better writers during the writing process. I know there are some projects I didn’t finish because I got 80% of the way through and what I was writing THEN was no longer compatible or cohesive with what I was writing at the beginning.
It’s tough stuff, writing to completion without tripping over yourself. I don’t get how the Big Famous Muckity Mucks do it. They MUST have magical literary resources, that’s what!