Yep, found the cause, at least for me. If you edit troops while preparing for a PvP match, then back all the way out of the PvP match, the last selected team in the edit screen (or the first team, if you collapse the list) will be set as your defense team. It only happens if you edit troops in the preparing to fight screen, THEN back out of the match completely. If you edit troops from the troop screen, you are fine. If you edit troops while preparing for a match then continue into the match, and you are fine. Just avoid editing, then backing all the way out of the match and you should be good (or make sure you reset your defense team if you do).