It is indeed "mirroring." I noticed my T doing this from the first session. It's affect attunement, the bedrock of attachment theory. The therapeutic dyad is symbolic of a mother/infant dyad. Since most therapy patients didn't experience a "good enough" mother in their earliest attachment, the therapist attempts a corrective attunement through mirroring (another way of expressing empathy). The goal being a secure attachment relationship that alters a patient's internal working model, resulting in a greater capacity to form/maintain healthy relationships outside of the therapeutic context.

I also would change positions just to watch my therapist "mirror" them. Once, he was so slouched in his chair that it was nearly impossible for him to write. I must drive him batty at times!