I wish I knew! Seriously. But they are often triggered by a situation or event I encounter in person, or sometimes by a news story, TV show, movie, or book that gets me asking “What if…?” And then when I try to write that what if, I don’t like it and it completely morphs into something else, but that spark was necessary to get me started, so… *throws up hands* Writers are weird. (Or if it’s just me, don’t tell me.)