Perhaps it was more of a personality test to see how you would answer and not them caring about gaps. Maybe they were seeing if you would come up with a crazy excuse and get defensive. Just a thought.

Edit - It is interesting without knowing intention people say to pass on the employeer. How many interviewers ask stupid questions not realizing they might miss out on a great hire? I think often companies forget that we are interviewing them as well. At any rate I think it's fair to ask in the interviewer why they are asking the question or what is their concern. Especially if the question already made you decide to turn down any offers.