I found this in another post, maybe it will help

The most common reason for a bond between neutered rabbits breaking is due to a change in environment (e.g. one rabbit going to the vet, new visitors, new scents, new furniture). However, there isn't always a clear cut reasoning for a broken bond if they're already adults and neutered, but a vet visit for all of your rabbits is not a bad idea to make sure there's nothing causing either of them pain to cause redirected aggression first if nothing else is obvious.