The first thing that crossed my mind was what some others had already said. The fact that maybe Butter can detect that Mimi is not well.
I find that interesting though that all the aggression happens outside and not inside. Just guessing here, but, could it be a territorial thing?

I am so sorry that this has happened and it must be so frustrating not knowing why, especially having it happen after 4 years of them together.

I wish you all the best of luck with this complicated situation and that it can be resolved in some way.

((((((HUGS))))))