Yeah there are a LOT of things that could be happening. But really it all comes down to the audience. For whatever reason YouTube decided to share your video to the audience that already enjoys that other video and it was successful, so they kept showing it. That audience has a tendency of watching and enjoying content that is similar not only in topic but also length, style... who knows what else.
YouTube Search and Discovery acts on a lot of things that we are not always privvy to, there is a lot at play when determining when and where to serve content. For whatever reason (i.e. a LOT of reasons) YouTube decided to test your video in that slot and it worked. Welcome to the wonderful world of Suggested videos! Now the trick is making another!