This question helps me get a full picture of the candidate's work history. What keeps the person motivated? Why, if the person has, did the person jump from job to job? And what is the key factor when he or she leaves?

The answer shows me the person's loyalty and reasoning process. Does the person believe someone always keeps him or her down (managers, bosses, etc.)? Does the person get bored easily?

There is nothing inherently wrong with moving from job to job--the reasons are what matters.