While you should definitely try to anticipate and prepare for grad school interview questions, you shouldn't stress unnecessarily. That you've been called for an interview by the grad school program of your choice is a good sign. It means that your application has passed the first few levels of scrutiny and that the admissions committee is interested in you. They now want to meet you and get to know you as an individual.
Put yourself in their shoes. They already know everything they need about your qualifications and your grades. They have your standardized test scores. They have letters of recommendations for you and they have your own assessment of yourself in the form of your statement of purpose or personal statement. Obviously, these documents have satisfied them enough that they now want to meet you before finally and formally inviting you into the program.
So if you were on the grad school admissions committee, what more would you want to know about a prospective student?
A typical grad school interview is not just about asking and answering questions. You get to see the campus and the facilities. You meet the faculty and current grad students in your department. You interact with the people you may be working with closely over the next few years. The admissions interview is as much about you finding out whether the program fits your needs as it is about the admissions committee making a decision about whether you will be a good addition to their program.
On paper, you match their requirements closely enough so that they have short-listed you as one of the few they will meet face-to-face and obviously, the program has something you like or you wouldn't have applied. This interview is about you as a person and the questions will be designed to discover your personal qualities, strengths and weaknesses.