I wonder if there is some underlying lizard-brain thing going on here.

If you "know" you are physically unfit you are quick to anger and aggression because you potentially need to act like that to not need to rely on physical fitness if it came to needing it? I.e. you need to deter others through your aggression rather than relying on fitness if it came to a fight?

Or alternatively the other perspective if you "know" you are fit you can keep the stress hormones low safe in the knowledge that if it comes to it you've got the fitness to handle a fight?

...Or perhaps none of that!

Interesting.