2 cats beefing

i have this 2 cats who are beefing with each other, i have 0 clue why. The victim is 3x bigger than the assailant (the assailant is a munchkin).

Story time.

It happened 2 years ago my family owns 4 cats, but the main focus is cat A (the bully) and cat B (the victim)

for some reason cat B is always left out, he used to be the most playful out of the 4 but no one wants to play with him except for us, owners. Cat A is short, fluffy but very territorial for some reason. Anyway, one day I was walking around the house, minding my own business until i accidentally stepped on Cat B’s tail. Of course, cat b was jumping and hissing, but what made it very weird is that suddenly Cat A started attacking Cat B out of nowhere and a huge fight started. Mind you, Cat A was at the door, Cat B was in the living room, so Cat A just attacked him out of spite

The another one of our cats joined in for some reason (munchkin too), and both of them started jumping cat B (i feel rlly bad at this point). So me and my mom tried to separate them….The cat that joined in is fine after like 30 mins but Cat A was aggressive for 3 whole hours. To the point we had to isolate Cat A in another room. Somehow it ended, but it didnt stop there

This beef is still happening till today, they cant be in the same room anymore, Cat B was just passing by but cat A just had to slide in a little hiss to get back at Cat B So Cat B is now alone, he often isolates himself and to be honest im getting kind of sick of their rivalry, i just want them to get along.

Anyone have any idea how to fix this?