Therapists have other clients who may be on a waiting list because they are in crisis themselves, or clients who themselves need to reschedule, etc. That is not uncommon. I know my therapist's receptionist was really good about letting me know if a spot opened up if she knew I was trying to get in sooner or trying to reschedule. So, I doubt this was passive-aggressive on his part. More likely, he took you at your word that you wanted to cancel and rather than having no client during that hour and losing that time and income, he offered that available time up to a client who was waiting.