Oh dear
Well first off are both Rabbits free of any serious injuries ? Puncture wounds in particular as these can be hard to detect when 'fresh' and they are the type of fight wound most likely to lead to abscess formation. So I'd have a very thorough feel over both Buns as a few hours after a fight a puncture wound will have 'hardened up' and thus be easier to feel under thick fur. Checking under the dewlap area and around the genitals in particular, also check that their eyes are not weepy/gunky. A fight can lead to a Rabbit getting a scratched/bitten eye/eyelid.
When a previously bonded pair suddenly fight the first thing I consider is could one of them be unwell in some way. For example if one Rabbit has a sore mouth due to molar spurs they may not be able to eat very quickly. So they may become 'food aggressive' and try to stop their partner from eating all the food. Or a Rabbit may pick up on a vulnerability in their partner and start to bully them. So a thorough Vet check may be a good idea.
Could something have scared the pair ? A predator outside maybe ?
Spring Fever can effect even neutered Rabbits, making them rather bolshy and stroppy. But somehow I am not sure that Spring Fever is the main issue in this situation. I may be wrong though, I often am !!