One cat biting the other - how can I change this behavior?

I have two cats that have co-existed in our home for nine years now. The older is 11 (Montana, female, fixed), and the younger one is 9 (Lucky, male, fixed). Both have been declawed. About three months ago we found that Montana had been bitten on her rear at the base of her tail on the right side. Obviously this was done by Lucky, as these are both indoor-only cats. It had become infected and abcessed, and she had to go in for surgery. We got that all taken care of, but then it just happened again two days ago. Again, to the vet we go. There have been no changes in our household. No new pets, no new people, no moved furniture, no changes. We have lived in the same house for 5 years now. I cannot figure out what could have triggered this to happen not once, but twice. Also, I cannot figure out how to prevent this from occurring again. We can't be home all the time to supervise them while we're at work (someone has to pay the vet!). Any suggestions, ideas, thoughts, help?

[spirit3]

2 of our male cats (both fixed) who'd lived in harmony also in same house for their whole lives together also went through a spell like this. It was very upsetting for everyone. It would start w/one staring aggressivly at the other and the fight would be on. It passed after a few months and hasn't recurred. what I'm wondering-we can hear cats fighting (or doing something "else") on street outside the house and I wonder if they can smell someone in heat and it's provoking them somehow. What's odd is we have 2 other female cats and it's only the boys who did this. We found it important to stay calm yourself and separate them and distract them w/a little treat or toy or something; I didn't like the water spray treatment, myself. Good luck, also there's some sort of calming product you can get online that's a fragrance of some sort you can try. Best of luck!!!