When I write, there's somewhat of a brain-disconnect. The back of my mind knows the story, and the front of my mind occasionally throws out a "Wait! Rephrase that..." but that's about all the action the front of my mind gets until the re-write.
So occasionally, I'll write eight or nine pages, and go back to check for any gross errors, just a quick scan, and have a moment where I think:
Now why in the hell did I put that irrelevant little tidbit in there?
If I can't figure it out right away, I'll leave it alone and leave the story alone for a bit, and then go back. If I still can't figure it out, I label it useless and take it out....
Sometimes though, I'll be re-reading, and the back of my mind will let the front of my mind in on the joke, and I suddenly know why I put that in, where it ties in later in the story, and what the significance is... all at once.
Does that happen to anyone else or is it just yet another example of how I'm really weird?