The mysterious Sandra plays a big part in this final story—did you know from the beginning what her role would turn out to be or was this ending a surprise to you? (Without spoiling anything, of course!)
With my shorter works like this, I tend to let the story evolve over time more than I do with longer stories. This means more discovery, as I'm not sitting down with a framework—the goal, often, is to practice other skills in my writing. (Things that my novel writing doesn't teach me.) In this case, I had ideas for Sandra, and some of those ended up going all the way through—but some I discarded over time. I'm not one who is "surprised" by my writing, however. I don't generally like that phrasing. Sometimes as you're working on a piece, you discover a thread or theme that intrigues you—so you dig into it further, then develop it. Sometimes this means the final piece of art doesn't match the outline. It's not really a surprise so much as a common side effect of the writing process.