tl;dr: they had no prep time, made real-time changes to the story as they shot it, and props and scenes were thrown together as they needed them. Jackson says that his winging-it—"making it up there and then on the spot"—finally fell apart when they came to shooting the big battle scenes. [via]
"I didn't know what the hell I was doing" — Peter Jackson.
The proposed narrative is that they paused shooting, got their shit together, then figured out how to make a great film.