Have Cat Questions? Ask a Cat Vet Online.
There are a couple of things here that you can do.
First, you should have the cats spayed or neutered as soon as you can if you have not done so already. This can help to ease some of the agression.
I also suggest that you help to relieve their anxiety by using an all natural product called homeopet (www.homeopet.com) and see if it helps any.
I do think that the behavior is caused by the interruptions from the other cats. Many times, perfectly normal cats will start to fight because an outside cat is in heat.
With time and some patience, they will return to being best buddies in no time.
Let me know if you still have questions.