It's strange, adult cats Tigger's absolutely fine with, but kittens she can, will, and does, bully.

I think it started as a way of protecting Ebony (cause Ebony was scared of next door's kitten for some reason) and even after Ebony passed, Tigger's continued it. It doesn't seem to be a protecting-territory thing either because Tigger invites adult cats back to the garden so she's fine with adults being around.