I don’t know what specifically makes someone an expert in whatever particular field, but in the context of epistemic trespassing that person is considered by others in and outside of their field to be an “expert” in their field, and then that person passes judgment on some question outside whatever field that they are considered an expert in. That’s why it’s trespassing and if they abuse their expert authority, which most people might not know that they did that, is why it’s wrong.