I wrote a novella by accident. Seriously. I've been asked, "How the hell do you write a novella by accident?" Perhaps you're wondering the same thing as you read this. It's a good question. I even have an answer for it:
You set out to write a short story about ten years ago. You write a very long "short story" right up through the climax. Then you get bored and drift away. You don't look at the file on your computer for a little over three years. Then you re-read the file (fixing all the horrible passive voice issues and adding several Oxford commas). You discover that it doesn't suck. It is, in fact, pretty damned good. So you finish it. It's a little freaky to find out it's done, because that means you wrote a novella--by accident. It really is a mildly disturbing feeling...and yet it is also so very, very cool. :-D