It is pure game theory. An aggressive person expects no bad outcomes from his passive victim. If they get a signal that their own outcome may be not that good, even marginally, this very often changes their behaviour.

That's why the advice to act submissively presented as "avoiding confrontation" is often the wrong advice.

You are not seeking confrontation, but you should signal that you are ready for confrontation. Stops aggressive behaviour very often.

Unfortunately this quite reasonable observation has been mangled by pop culture and memes into "be aggressive yourself".

It also fails to account for there being different sorts of aggressive people.