I have heard this happens sometimes especially if the male has continued to try and mount the female, she just gets fed up with it and tries to put the male in his place by nipping at him.
Has she drawn blood or really hurt him? It might look as though Fin is scared but he may just be being submissive to her. I'm no expert but I am just trying to think of a reason why the pair seem to have fallen out. If it were me I work at bonding them again slowly rather than splitting them up unless of course they were really fighting and hurting each other.
I really hope things improve for you all.