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.

You assume that was an error, based on what?