When Stories Explode

This has...happened to me a few times. I'm sitting there, fooling around with something, and I think, "Oh, hey, that would make a great story!" Sometimes something comes of it; sometimes it doesn't. I never know when it's going to happen, either. I might be watching a movie, and start to think, "Hmm, what if?" I might be reading a book, and wonder the same thing, or playing a game...or listening to someone talk about a game, which has happened before.
But when it seems to happen the most is when I am sitting down already writing. I start to put in backstory, mention a character in passing, and suddenly become more interested in the passing character than the main story, so I have to move to a new document and write down at least the idea for this new tale. There is a mysterious tower with a strange inhabitant--friendly or unfriendly--and suddenly, I want to explain how this tower and the inhabitant got to be the way they are. Someone finds or receives a storied item, and I want to tell the item's story.
Sometimes, I am able to quash the urge and go on with the story I'm writing. Sometimes, I take down a note and then write the other story later. Sometimes I can't make up my mind which to do, and that's when I leave the computer and get some coffee--or I don't leave the computer and start playing Spider Solitaire.
But what happened recently to prompt this post was a time-write. The last one I posted, in fact. I've had time-writes that looked like they could branch out into something bigger. I like those; they're doing what they're supposed to. But this one exploded. It didn't just look like it could become something bigger. It did--right away. Almost as soon as the timer went off--taking a few minutes to rest my cramping right hand--I got on the computer and entered in what I had written. And kept going. And going. About the time I went to post my comment, I had to go back to the notebook to find where I had let off, because it had gone from half a page to almost three.
It has gone from a page-long sword-and-sorcery exercise to an epic novel, involving two armies of villains, an order of knights, and at least two battling supernatural entities.
I am trying to keep it from ballooning any more than it has...but this one seems to have the bit in its teeth, and I may just have to hang on for the ride.