As a former psychologist I can understand the change in language from transsexual to transgender since the chromosonal sex isn't actually changing just the outward appearance to correct the self-image, but I don't understand people who get super defensive or worse aggressive about people whom still use the old terminology.  In fact, if I was to be transitioning myself, I would probably prefer to think of it as being transsexual and actually changing my sex, if the science would allow it.  Alas, it does not.

The trend of getting angry over people misusing words is big with many transgender people.  It comes from an authoritarian mindset, and not a very productive one if you ask me.  If you anger people or frustrate them with riding their ass about something, they are not going to be very ameneable to your demands.