I've had this happen and, in fact, recently lost most of a session and was so confused and distressed as a result that I saw my therapist the next day for a extra appointment. She believes (for me) that it was a physiological response--a nervous system flood of stress hormones (or something like that)--and that it must have been from something she said (a word or topic) that triggered that kind of response. Neither of us knows what set it off, so it's just something to notice and try to identify.

I do think that therapists get sketchy when they think a client is reacting to them or to something they say. The test is in how they respond to being the focus. Hopefully, yours will prove as steady as he has always been and you will get on track to deciphering what the trigger was and then do the work of softening it.