I am not sure if it is possible to show symptoms of BPD only with your therapist, but maybe because it is therapy, it is more likely that certain buttons will be pressed and therefore more likely that these behaviours will be there. Or maybe we are more aware that out T's can dig deep and we try to defend ourselves.

I am good at putting up false fronts in most areas of my life, sometimes in T it is safer to let the guard down a bit and again maybe this is why certain behaviours are more likely to show themselves.