The playoff winner from each league promotes, and 4 teams relegate. It is quite possible for a bot team to promote, and have you relegate in their place. In your particular case, there was actually an active team that promoted, but that needn't have been the case.
The USA doesn't have enough active teams to fill 4 divisions, so there are lots of opportunities for teams like yours to be relegated into isolation. BB could be smarter about how promotion and relegation are handled so that the active teams are concentrated in fewer leagues.