I agree there is no best way to answer that question. Think about it in this way: all your background is shown on your vita, why they still ask that question?

From my experience, I think you can't just summarize your experience or background. The reasons behind it are the things people really interested in.

For instance, if you have changed your major in school, why? If you have changed your jobs, why?

There might be thousands of reasons behind it (e.g. family problem, money etc.). But it is better to "figure" a reason related to the job you are applying or related to your career goal. Make it reasonable!!