I wrote a story, but afterward came up with the idea to write another telling how pieces fell into place for the first one. Would that average reader prefer a prequel, or just have it come before the original story?
Nothing wrong with prequels as long as they (a) actually do fill in some blanks in interesting ways and (b) don't contradict or undo anything introduced in the original story or at least do it in an interesting and justifiable way.