My T does this, too. I believe it is so that you won't look up and find the T staring at you, because that could easily feel very uncomfortable. It's a way of giving the client a bit of space, is how I think about it. I used to think that my T might be missing what was going on with me if he wasn't looking at me, but he is very good at noticing stuff anyway, I have found.

It's always okay to ask a T why they act like they do.