Why if people understand me do they reply in English?

Living in Denmark and I speak Danish quite well. Many people friends etc speak Danish with me all the time and its cool but so often when I meet someone I don't know they will answer whatever I say in English even though I do speak in Danish. It's like they hear I have an accent and assume I am speaking English even though I am speaking Danish and they understand me. What is it. I guess I shouldn't care but now I am looking for a job and it seems to be a big hindrance