I bet some T's do experience anxiety over letters or emails. Especially if they are newer in the field. Who knows what a client could have to say? Especially if the T is worried about the client for something going on in his/her life, or if the T knows that a session did not go well recently, this may make them a little tense until they read the letter.

Personally, I've had the experience of giving my T a long letter while in a session and having her read it in front of me. I told her a lot of open, frank things and was more honest than I usually am. I think my T was stunned about the things I told her, and she acted a little awkward since I told her so much at once. I think she finds it easier to respond to things in the moment instead of responding to an entire letter full of things. So perhaps my T was a little uneasy about how she was going to handle the letter as she was reading it. I could of course be completely off and have read her reaction wrongly.