It's literally the only thing people are likely to put on their CV that's a possible indicator the person in question might not actually be a good fit for certa

It's literally the only thing people are likely to put on their CV that's a possible indicator the person in question might not actually be a good fit for certain positions. Of course it's their business to ask. There are perfectly good answers (which can include "it was a six month contract", although that stuff can be checked...) and there are even answers with the right amount of objectivity/detail which give the impression the reason why the employee left or wants to leave makes them a particularly good fit for this new job, but there's also a good chance the employee's answer raises red flags about their attitude/ability/expectations that they wouldn't give off in response to other questions.