Trying to interpret what has got a couple of cats scrapping for no good reason that we can see can be quite daunting. All I can do is to make some suggestions that you might look into. Could either of the cats have any medical issues that you can't see? Cats have an uncanny method of hiding medical problems that we can't see, but it can be an issue with one of the other cats. Has there been any changes in the environment either inside, or outside? Inside: like new furniture, paint job, repositioning furniture – as you can see the list goes on. Outside: has there been any construction around your dwelling or close by? are you at ground-level? If so has there been any animals in or around the yard (their territory) that the cats could have seen that would have started the upset. Did anyone lose their temper with one of the cats? Yelling, hitting, or kicking can cause this type of problem. Did you have company in for a meal, or for a few days before the problem started?
As you can see things of this nature can be extremely hard to chase down. What I'd be doing would be to rule out the health problem first and then try to figure out what stressed the animals. I really have no knowledge on how you get peace between the two animals working again. Hopefully somebody else can add their opinion or shed some light on it.