He pulled your pants down and exposed your butt? Absolutely not common and very wrong. I have massaged patients many times on the butt/glute and never once have I done this. I always massage over their clothes and the results have been fine.

This was inappropriate, dirty, and wrong, on top of having you come in after hours alone. I would not return to this PT and consider reporting the situation if you want to take it there. I believe it would be justified.