When an idea for an army just clicks into my head. Theme, modeling, list, color scheme and background. When they all come together into something I just need to do, that's when they start getting built.
I am also usually working on concepts for a new army while building a current army. Like now I am working on my Night Lords (counts-as BAs) and have everything figured out conceptually like color scheme, background, overall feel of the list, so now it's just build-paint with very little head scratching. So I usually end up thinking about the next project...Sons of Medusa.
Strange, but not a Stranger.