Looking at your resume, I see that you were at your previous position a fairly long time. Why did you leave?

Having a long tenure with a company can be a good thing, but it also begs the question of why you need to make a change. The interviewer is trying to flush out any issue that would have forced a change after such a long period of time. It is best to be positive when answering this question and not belabor any dispute or grievance. Understand why the hiring manager needs to ask these kinds of questions so that you do not get defensive when having to provide an answer.