And it's a harder question to answer than I thought it would be, especially if I was trying to avoid sounding pretentious. :) No one's *happy* if you turn a book in late, but, as I said to her, it's something like "you can't put art in a box". Writing isn't a one size fits all occupation, and the weirdest things (or not so weird) can derail you. I didn't write anything for a month when the dog got sick and died. (Which is not why my book is going to be late. That's a whole different scenario, but still.)
So it happens because the dog dies or feedback is late in coming in or you hit a wall with the story or the project that's been lying fallow for eight months suddenly goes into overdrive or the day job is overwhelming and you simply don't have the energy, or, and this is a big one, I think:
If you have a 9-5 job, you very rarely, if ever, hear somebody say "I don't have time to go to work today." If you are an artist of any sort, and reliant on your own discipline, and working at home, there are always things to do. Shop, clean, cook, go for a walk, whatever. Half a dozen small things can become "I don't have time to work today" very easily, and that just doesn't happen if you have a day job. You might, once in a great while, take a day off work to get things done that are desperately important but cannot be done on the weekend, but people do not take the day off to catch up on laundry, generally speaking.
It happens if you're just plain a slow writer and expect to finish something sooner than you do. It happens--this didn't actually happen to me, but it came close: with QUEEN'S BASTARD, I had a synopsis that turned out to be about a book and a half's worth of material, and I thought it was one book's worth. I got lucky and didn't slam into a wall over that, and I know that doesn't *sound* like a problem (just cut it in half!) but it's actually a fairly massive thing to deal with, creatively. It can mean changing the whole way you think about the book, which can be very, very difficult to do (said the woman who is currently in the process of doing a massive revision which requires thinking about the book in a whole new way).
I've read a story of one writer who was way overdue on a book and went apologizing to her editor, who said, "It's okay. Babies won't die. Be calm," and that's part of it too: turning a book in late is not an end of the world scenario. Another author I know just turned in a manuscript that was more than three years overdue, and that person is a consummate professional. That book, though, had just plain been impossible to get through. JK Rowling took forever getting the, what, fifth? book out, in part because somebody was suing her over something, and that sort of life event can just wipe out the energy you have to do the job with.
So it does happen. Most times, you're probably only going to be a few weeks late, and publishers usually work that kind of lag time into their schedules, because writing /is/ a creative endeavour, and try as you might, it's not easy to quantify it. When the lag times get greater than that, well...when you get right down to it, the book's just going to be late. That's how it is, and the truth is, babies won't die.