We noticed last week that the number of players using single-troop defenses in PvP was growing.
While it was a reasonably rare occurrence, we were quite happy to indulge it as we considered some better solutions for PvP in the future. The plan was to eventually make some really neat updates, but only after we'd had a chance to fully think everything through... I'm not a fan of quick fixes (necessary though they may be sometimes).
Anyway, we noticed that there were some pockets in the game last week where players could neatly cycle around single-troop defenses without ever playing a real game of PvP against a properly constructed team. Needless to say, that's not really the spirit of the game as we intended, and is probably harmful to the game experience in the long-term.
So for now we've opted for a quick fix, that should actually be reasonably harmless and barely noticeable to most players. Any single-troop defenses, will be replaced by another team in the defender's collection for now, until we can investigate & test a more thorough way to solve the problem.
EDIT: We're also aware that you can set EVERY SINGLE ONE of your teams to one troop when you're not actively playing. I don't think we'll see a lot of this, but if we do, we'll just be adding some code to the server to do something like saving the last full PvP team you won with, and using that (or, worst case, we'll have the server construct a team for you)