New aggression in multi-cat household

Question
Photon, an extremely good natured 8-year-old, neutered male whom we've had since a kitten, shares the house with two two-year-old, neutered female sisters we adopted as kittens. All three are 100% indoor cats.  We've all been living in complete harmony for 2 years until about a week ago. Suddenly, one of the females started tearing off after Photon, screaming, hissing, spitting and cornered him under the bed. I thought it was a one-off happenstance, but the behavior has escalated. Now both sisters are in on the act.  All Photon has to do is be seen walking in the room and the girls start their aggressive posturing, and without intervention, the screaming, hissing, spitting, chasing begins. It's affecting meal time, play time, and as of last night, my sleep time.

We can't figure out what's causing the problem. Photon hasn't shown any aggression at all towards either of the girl cats.  In the past (but not recently), I have seen him display some sexual aggression (nape bitting), but it never got serious enough to cause the cats or us too much concern. I don't want to be  too quick to think that's the source of the problem.

It's almost like the girls see him as an imposter, or that he made one of them permanently enraged with him.

We're looking for insights.

Thanks,
Jean  

Answer
Hi
This could possibly be one of two things. Firstly there is something known as trauma aggression. This is caused when something happens that makes a cat particulaly scared and they take out this fear by blaming another (often innocent) cat. As an example of this, say a car back fires and startles a cat, they then attack the cat sitting next to them, almost as if they blame that cat for the noise and consequently the fear. Strange i now but this is common. Usually when this happens the problem will go away of it's own accord over a few weeks, when the fear and the association has passed. If it doesn;t then the cats have to be reintroduced as if they had never met before to help rebuild their trust in each other ( i have a page about this on my website in the training pages).
The other possibility is plain old fashioned territory rights. Why this should suddenly occur can often be a mystery, it may be that the male cat has been over stepping his place in the group some how and the female cats are trying to assert their place and territory. Again there isn't much that can be done and usually cats will sort out their own differences and settle back down again.
I hope this helps a little and i hope that all your cats settle down again soon with each other.

Best wishes
Kate