Gender and separation of humans based on those lines have been a thing for longer than humans even called themselves “humans”, and transpeople threaten that. So it triggers a flight or fight reflex in people who aren’t used to the concept.

I find when people know me for awhile and THEN find out I’m trans, their reaction is less “I’m scared”, and more “That’s neat”, but if people find out I’m trans before they get to know me, they will be hostile the whole time.