This happens for various reasons. #1 The question being asked is around a hot topic or something most people can relate to or have a strong opinion about. This will cause them to post an answer. #2 If the question is under a category few people are following then they won't get an email notifying them a question has been asked. #3 If the person asking the question has large number of followers those people are notified when there is a new hub or question from that writer. It's been my observation that "gender relations", "current events", and controversial issues lead to more responses.
Last but not least some questions have been asked so many times such as "What is love?", "How do you know you've met the one?" What would you do if your mate cheated on you? or "Do you believe the world will end this December?...etc After awhile people who are following those topics get tired of answering the same questions! :-)