You say that the therapist is not the current target of your anger - that suggest you have an idea what it is then.

I did not feel like angry messaging my therapists unless they did something I really disliked and failed to own up to it. It was mostly one therapist and how he operated. Sometimes the proportion of the anger felt too much relative to the Ts act and impact on me though. Usually I found they were things I am particularly sensitive to in general because I have high standards in those areas. Sometimes also things I did myself when I was really not in a good place mentally. So it can be in part projection / displacement, but it was never just out of the blue and without something real about the T.