Rabbit fight

Question
Hello Andrea,
I have two male rabbits, 4 months old or so. I'm not sure if they're brothers because they don't look alike, but the pet store did receive them from the same place, so they have been together their whole life.I've had them for about 3 months and they were the closest friends I've ever seen. One was clearly dominant as he cleaned the other a lot and mounted him sometimes and is just overall more active. I was visiting family and had to return home  today, so they took a 5 hour car ride with me. They have traveled for this long in a car about 6 times or so, and although they won't eat or drink during the trip, they act fine afterwards. When we got home today, I put them in their playpen as usual and everything was great. About three hours later, I hear horrible noises and find them literally rolling around on the ground fighting. I couldn't believe it. I grabbed the aggressive one and he started screaming this shrill sound which supposedly they only do in fear or death! I can't remember though if he was already screaming before I picked him up or not. This is so bizarre because they were very, very close. I separated them for about an hour (and they both seemed fine, although the one that screamed seemed a little more shaken up). I just tried putting them back in and sure enough, they were circling, staring at each other, and started to almost nip a little so I grabbed the aggressive one again. Was this nipping normal or would it probably escalate? I don't know what to do! I only have one cage for them but clearly, I can't put them back together. Why do you think they would all of a sudden fight like this? I did give one of them a toy about 30 mins before but they share everything. Maybe the stress of the car...? Please help me out!! Thank you for your time!

Answer
The one thing you don't say is if your boys are neutered or not.

What I think may have happened is that after your visit they decided that they had to re-establish their hierarchy or had some stress to work out.  Even though it is their home and they have left many times, this time was different.  There are a variety of things that can do this; new person, new animal, stress, or changes in scenery.  If they are not altered, they are probably both in puberty and have more hormones going through their systems.  This makes them more territorial and they will start to engage in more territorial behavior.  Fighting, leaving droppings or urine around to mark territory, and spraying would all be indicative of territorial behavior.  

If they are not altered, the first thing to consider is to get them neutered.  One the hormones get out of their system, it will make it easier for them to get back to their relationship.

But now that they have started fighting, you'll have to keep them separate when you can't be there to supervise.  This might mean letting one run around in the room while the other is in the cage.  If you can grab a second litter box any kind of food dish and something for the water.  (you can fasten a bottle to the outside of many cages).

When you are there, the first thing to do is to watch for signs that fighting may occur like tail up or ears back.  This would be a good time for a squirt bottle to squirt at them if they start to work up.  Also, since they've been friends before, you can try placing a bowl of their favorite food out and see if they will eat in the presence of one another and interact without fighting.

Another method you can try is the "stressing" method.  Some people aren't comfortable with it.  I usually do it as a last resort and won't do it with easily stressed rabbits.  This site has some tips for bonding and there is a stressing section and tricks section you can try out:
http://www.rabbitnetwork.org/articles/bond.shtml

Don't know if the car ride method will work for you obviously, but you can try the other methods.

Don't despair, they liked each other before. Sometimes things cause rabbits to have down periods in their relationships but they can work through it in many cases.