Reply and trying to keep it short...
Your cancel collided with an opponent being selected. A glitch. Computers aren't perfect, and the internet has lag. If you press the cancel button, the risk is on you that it will cancel out correctly.
Your opponent forfeiting and hosing your game is something that plagued me around 4 showdowns ago. It happened because my game froze and/or error messages were thrown and I responded by leaving the round and trying to re-enter. Did not work, and rounds were given +3. I then resolved myself, the next time an opponent forfeited, to just sit and wait - do NOTHING. And that worked. And has continued to work through the last 3 showdowns I've played.
If you freeze up and suspect your opponent did something naughty, or if you get a message stating something's wrong - do NOTHING. I know it feels like forever, but just wait. Eventually you will get an in-game pop-up that you can continue without the opponent. Through multiple elimination and final rounds, I've had probably 10 or more rage-quits and/or straight up forfeits, and have not had anything bad happen at all. 100% problem solved - so far, and YMMV.