Some people are. There are people who intentionally use the wrong pronoun, exactly because they reject someone's transgender identity. We cis-gendered people may not notice this, but to transgender people, it is every-day reality to be confronted with someone who insists on denying their identity. Consider the stupid bathroom bills in a couple of US states for example.

So against that background, it's entirely understandable that they react more emotionally also to accidental mistakes. That doesn't mean overreaction is a good idea, but it's a backlash against a long history of having their identity denied. If we don't like the backlash, maybe we should work harder to address the original hatred that it's reacting to.