You have been lucky if you have never had that happen to you. It isn't a big deal, it just annoys me to see extra save files show up in my list of saves. I always set the auto save to never, but I will still get an auto save file at random times. And I do mean random. I could play for a few days and everything is normal, then next time I play for a bit, I will get an auto save file stuck in the save list.
I could fire the game up, craft some junk, get plowed by a few random monsters, quit the game and every thing plays nice, next time I fire the game up, same process with the addition of entering a dungeon or two and I get a pile of quick save files and occasionally the game will slip an auto save in there as well.
The only thing I can think of is that the game is auto saving whenever I enter a dungeon (or just start the game to check/test something) and then quit back to the main menu or just exit the game.
Dunno what's going on and it isn't a big deal and isn't crashing the game or anything like that, it is just an annoying little thing that randomly happens.