Why did he do this?

[ByeByeLilSebastian]

Old cat is 17 and very fragile, young cat is 7 and knows old cat is boss.
We have a neighbourhood bully cat that keeps coming in out garden to terrorise our cats, he's horrible and vicious and young cat does his best to fend him off.
They've just had a huge fight now, I has to step in and chase bad cat away. Old cat was inside but watching from the window.
I brought young cat in and then old cat went for him! He's never done that before, why has he done it now when young cat is feeling vulnerable?!

I would presume he's just grumping about the fighting and feeling a bit unsure of himself. Cats tend to lash out if scared. Young cat might smell of mean bad cat too.

Lots of cuddles and reassurance for both (if they like that sort of thing) or even a bit of a shared treat, bit of ham/chicken /tuna each.

That's true, he probably did smell a bit and old cats eyesight isn't the best.
I can't get this bastard cat to stop bothering my 2 though he keeps coming in the garden looking for a fight.

Maybe get a water pistol for bully cat, or if you have a friend with a dog that your cats are ok with, have it round now and again.

I'd say OldCat is re-asserting himself as boss. Maybe a bit of pent-up aggression after watching the fight.

I agree with the supersoaker water pistol treatment to bully cat. I wouldn't mind someone doing that to my cats if they were being a nuisance.