Lengthy Backstory:
Adopted 2 6mo kittens in 2015.
1 - Bounce - made himself “to home” by August.
The other - Cheeto - lived in my basement for 4yrs. Only moved upstairs when I needed work done there & livetrapped him.
He & Bounce became BF(eline)Fs.
Often velcroed together napping.
4yrs later, he succumbed to FIC.
That was July '23, Bounce was an only until October. I went with a friend to a shelter & found they had a Srs for Srs program that adopted older cats to older people Free. Vaxed, neutered tested for FeLu/FIV & microchipped.
A week or two later I took home Stripes - 11yo skinny tiger guy. With some renal issues.
After some initial Kitty Opera they settled & while never superfriendly, they were at least cordial to each other.
Then in October this year, Stripes crashed & I lost him.
I donated the nearly-full case of the pricy Royal Canin kidney food to the shelter & on a whim(?) asked to see any Srs they had…
Resulting in me taking Milo - 6yo - home a week later.
Now, nearing 2+ months in, Milo seems settled, but Bounce… 
Still singing The Song of His People if he so much as sees Milo.
Who, TG, ignores him.
The LR is now Bounce’s territory, kitchen is Milo’s, hallway to the BR is the DMZ, BR is Milo’s.
I am Switzerland 
No engagement or paws thrown, just noise from 8# Bounce & 
from 12# Milo.
On to my question:
A couple days ago I heard hissing/growl from Milo’s usual perch in the kitchen (my desk chair).
Thinking it was Bounce, I checked & found Milo fighting his own tail 
Since then he’s done this a couple more times.
Always when he’s on “his” chair.
Chair also holds one of those feather-on-a-stick toys he plays with while lying on the chair.
Still his usual calm self otherwise.
I try to give him some Me Time daily, sitting in the chair with him on my lap. He totally relaxes, turns bellyup, purrs. He also accompanies me everywhere except the LR. Though he does stroll in there occasionally, ignores the Bounce Siren & sits in the window.
Not my Shadow, but supervises my making the bed, taking laundry to the basement, etc.
WTH?
Google suggested self-agression can be from tension, a skin disease or pain.
Milo’s coat is in great shape, nothing I can feel or see at skin level, no sign of pain, eating, using the litterbox, etc.
Some sad-sounding vocalizing, but rarely.
Whassup with the Chair Deathmatch? 
Power Hour!
