You might have been auto-joined when you went to find a match. Well, when the game is about to start, there was a countdown, and it might have been finished when you finally entered the match. It happened to me before and I think that this is what is happening.
A support case was created for you and you should be contacted shortly by a Customer Support Representative, if you haven't been already. You can view and update your case at the Activision Support website. After logging in, go to "My Support" then "My Cases".