I would also lean toward coincidence. If something happens only once, it's important that you not jump to conclusions. For example, if I see an apple in a lucid and I wake up. I might conclude that apples make me wake up in dreams, and this very conclusion leads to a self-fullfilling prophecy where I actually DO wake up every time I see an apple. A better idea is to set aside this one experience as coincidence for now, and see what happens in the future.