Well, most of the time an idea just hits me an I go with it. Sometimes I'm working a specific idea or type that I want to develop. Then sometimes, especially when I'm in a meeting or waiting on something, I just need to draw just to get through the time.
Some are being developed for games or stories. Many are "just cause" type of sketches. When I post those "just for fun" I come up with a name on the fly and through them out there.