I haven't read the push/pull thread, so I might be misunderstanding this one, if they're linked.
Isolation for me is often about walking away before anyone can get close, never accepting invitations, not keeping in touch, returning a call, seeming disinterested when I do communicate. So if I can have passive isolation, I will.
Isolation, ie pushing people away, has happened when that person has been involved in triggering a more severe reaction. I needed to be allowed to isolate and get myself to a better place. I would have liked to have walked away quietly, and have had someone waiting for when I got better. But people create all sorts of 'hidden meaning' to explain a friends isolation, When I'm in that place where I need to isolate, I'm so shut inside my head, that it isn't possible for my mind to find an explanation.