To avoid eye contact.
When humans don't know the answer, we tend to avoid making eye contact so that our un-sure-edness (I'm aware that's not a word) doesn't get conveyed. What people fail to understand, though, is that you're still letting people know that you either didn't understand the question or don't have an answer by the lack of eye contact.
Either grow some balls or lie quickly while maintaining eye contact.
http://youtu.be/wFAzSN36c08
