Sometimes, writing a book is a linear thing. I start at the beginning with a solid outline (I always outline in a lot of detail) and proceed straight through to the end. Inevitably though, I discover things about the characters, the plot, and the theme that I didn't know in the beginning, and I have to go back and retcon things to make it fit. That's pretty straightforward. It's also rare.

Sometimes a book goes completely off the rails, and I stall about half-way through, flounder around, and finally have to go back to the beginning and put everything in order before I can go on. That is really frustrating.

Usually, though, writing a book is an inside-out, timey-wimey sort of ball of narrative threads all twisted up like Christmas lights that have been in storage for a few years. In addition to the main plot, there are all these other subplots, side-plot, and digressions that I have to connect to each other and resolve, but I don't want to go back and start over at the beginning because I've got momentum on my side, so I keep going and just write through the ending, wrapping everything up in the way that seems most satisfying. THEN, I have to go back and retcon the beginning so that the ending makes sense. Until I know how it ends, I can't know how it should begin.

Addendum to the above: I have tried writing endings first, and though it is sometimes helpful, I find that I don't know what a book is about until I write it, and so I can't really wrap everything up because I don't know what everything is. The only way out is through.