It usually occurs when I least expect it, a sudden flash of inspiration and an equally sudden lack of writing implements.
I call it the Floodgates Phenomenon.
Let's take a guy, his name is Jack. He's a stock boy at the local grocery store but is also an aspiring writer. One day, while ripping open cartons of Corn Flakes and situating them on the shelf, he thinks of a great opening line to his new book. He looks up at the clock. Crap, it's forty-five minutes until his break time - a near-eternity for this starving artist. There's no way his boss, Manager High Pants, is going to let him take a break early, not with three more pallets to sort through. He keeps diligently working, all the while that great opening line is just tumbling over and over in his mind.
Finally, it's break time. He grabs his notebook, steals away into the corner of the breakroom, and writes down that opening line.
Phew! "Thank God," he says to himself. "If I would've waited much longer, that one would've gone away."
Then, once that line is down on paper, the next paragraph flashes before him. He also writes it down. Then, more and more come to him. His pen flies across the paper and he's just pumped at the progress he's making. Before he knows it, his entire break time flies by and he's written two or three pages.
Ever felt like this? I'm sure you have, even if you're not a writer.
The Floodgates Phenomenon seems to occur when this "flash" shines forth like a lighthouse beacon and you can't see anything beyond it. Then, once you've committed it to paper or word document (or whatever writing surface you can find - although I don't recommend the bathroom stall, if but a last resort and you don't mind having your idea next to pleas for who to call for a good time) more floods your mind.
This happened while I was planning what to do with this blog. I thought I'd have twelve decent blogs and that'd be it. Now I'm past forty and I have Post-Its with scribblings of at least twenty or thirty more. And do I think that's it? No way!
It may happen because what is first and foremost in your mind dominates with such authority, that the rest waits patiently behind it, not even announcing their presence until it's free to do so. That's the way writing is with me. As soon as I write whatever is dominating my mind, whether it be the next scene of a novel or a new short story or blog, the floodgates open and more ideas pour out than what I originally thought.