Maybe I'm wrong but the dex entries may give you a clue:
Sword Five of them are troopers, and one is the brass. The brass’s orders are absolute.
Shield The six of them work together as one Pokémon. Teamwork is also their battle strategy, and they constantly change their formation as they fight.
Basically, they listen and work together well. This is more a theory than anything, but from natures and characteristics that boost sp def, it's been told that sp def is kind of doing what you think you should do, not not running from that. Take careful for example, which means to take care in what you do, or do what is safe. You aren't doing something risky, doing what you think is safe.
This funnels into two things that may help: one, you would be Intimidated by a mini army, and two they have their own commander and can teach you not to always follow what someone else says, because you beat them and they lost because they kinda listened to the commander.
Another thing: its HA is defiant and a good sp def characteristic is strongly defiant. Perhaps this gives a display of what doing what you think you should do. And yes, I know this contradicts the other one, but in reality, the more theories you have the more likey we are to be correct. Sorry if this was top long, but I hope I explained something!
