Obviously if the interviewer asks me why I chose my major and I answer, it wouldn't be appropriate to in turn ask them about their major. However, if we're talking about hobbies, what I like to read, that kind of thing, is it appropriate to occasionally ask the same of them? I'm naturally very curious and love learning about people so without consciously thinking about it I would probably ask these questions, but if it's not appropriate I will make sure to be aware and avoid it.