Long shot, but you may have contructed things in a way that blocked a path that the AI uses to walk.
IF you were allowed to create these barriers, you could effectively block off important areas from any enemies.
To prevent you from cheating this way, the game will spawn in enemies anywhere you have a blocked area (an area that people can't walk in).
Generally, this will result in massive base damage and explosions, it could theoretically take out your lockers that fast and drop you back down to 7.
Again, it's a long shot, you didnt give a huge amount of info, but it's one of the only ways I can think where minion capacity could be dropping so rapidly.