it's common to most NT's I think... or T's in general, actually. reasons/motives for it vary hugely from individual to individual, but as a general tendency, I think most T's tend to carry some kind of baggage around that causes a knee-jerk response to the phrase 'open up' and the word 'intimate'. In my case it's not so much that I don't want to, or that I fear losing autonomy, as just that I have had lots of experiences where I've overridden my emotional baggage and taken the chance, only to be abandoned later.
So, fear of abandonment can be added to the list of possible motives for fearing intimacy. I remember a poem I read that was written by a teenager with Asperger's Syndrome. He talked about how, at great pain and personal cost to himself, he worked and worked at trying to build a bridge between himself and the rest of humanity, on nothing but trust, listening to the people who promised him that if he stopped isolating himself and opened up to other people, then things would be great, he wouldn't have to feel lonely any more. Well, in the poem, he did all this, and then once he built his bridge 'with tears for supports, blood for rivets and pain for ropes', and after taking a painful leap of faith to cross it, he found there was nobody there on the other side.
That poem resonated with me quite strongly at the time as it was how I've often felt when I've listened to people lecturing me about opening up, then I've taken the leap of faith and then the person who was so desperate to get me to do it has not been there to show me round this new territory. So I've just dashed back to the other side again, burning the bridge as I go.
Each time I've opened up to somebody, I've had to build that bridge again and it's been harder every time in light of previous experience. So each time I've been let down, it became less likely that I'd ever try again.
What changed was that I had to alter my expectations and motivations for building it. But that in itself is an ongoing struggle. The temptation to just withdraw permanently is always there in the background and sometimes it comes further forwards.
It's funny how my E can create a false impression of my not having withdrawn; it pushes me out there, but it doesn't demand that I attach. The temptation to withdraw doesn't manifest itself the same for an E - the withdrawal is inside, whilst with the INTP it's externalized in a more literal sense, staying in, not talking to people, becoming a recluse. In my case, I just spend most of my time alone in a crowded room, when this happens. It can be harder to get support because it's harder to recognize when an ENTP has withdrawn. People say 'you've got loads of friends though, you're always out and about!' and they think everything's A-OK.