As any student of cause and effect can tell you, a coincidence is when two or more events happen either simultaneously or in sequence, without any sort of obvious (and in most cases, inobvious) causal connection. Generally coincidences surprise us because, naturally enough given their nature, we just weren't expecting them. Most people know that sometimes things just happen at the same time. But some people just refuse to believe it. There are, after all, reasons people say "where there's smoke there's fire." They also often say "There Are No Coincidences." Compare Chekhov's Gun.