This is unfortunately a very common problem, and is very stressful for all humans and rabbits involved!
The most common reason we see on this forum is that the rabbits were living together as babies, and one or both rabbits hit puberty.
Baby rabbits do not truly bond (although this is a common misconception). We are not sure where this myth comes from, but new rabbit owners are often told that babies are easier to bond, so they get two babies. Once hormones come into the picture it very commonly causes fights to break out. This can happen either before or shortly after rabbits are castrated (as both situations mean a change in hormones and behavior).
In these cases, the rabbits should be separated completely (no time together at all, but they can be housed as neighbors), both should be spayed/neutered, and once hormones are settled, they can be bonded.
Do not despair though, you are not separating a bonded pair, as they were never truly bonded to begin with!
This can also occur in pairs where one rabbit is already castrated and the other is not. Hormonal fluctuations can trigger fighting, which is why the general recommendation is for both rabbits to be castrated to ensure a stable and lasting bond.
In a bonded adult pair where both buns are spayed/neutered, another common cause of fighting is that one bunny is sick, or went to the vet without the other bunny. Illness or a trip to the vet can cause one bun to smell differently and trigger some disputes. Bringing both buns to the vet together can help with this. If your vet won’t allow both bunnies to be there together (in the case that your bunny has to stay there for care for a while), just bringing the other bun with you when you pick up the bun from the vet, so they ride home together in the carrier, can be enough to recement the bond.
There are other reasons for fighting in rabbits where both are adults and both are spayed/neutered. If the bond is new, the bonding process could have been too rushed. A change in the household (moving, new pet, new roommate) can also trigger some scuffles. In these cases, some bonding sessions in neutral space can usually repair the bond relatively quickly, as long as the fighting was not serious.
If the squabbling between adult castrated buns is not serious (no injuries or hard biting), often you can supervise them in neutral territory and let them resort out their dominance, as you did in the bonding process.
However, if the fighting is bad, you should consider the bond broken. In this case, you will need to separate them, allow them to cool off for several weeks to allow the hurt feelings to repair, and then restart the bonding process.
Give your bunnies some extra cuddles, take some deep breaths, and don’t panic! Most bonds can be repaired with some time and patience.
. . . The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.