The interviewer wants to know why you chose to move on. A great response would be to say, "I was ready to take on a new challenge," or "there was no room for growth in that position." It's always best to draw attention to what you are moving onto rather than focusing on what was wrong with what you left. It's understandable to want a new challenge or to desire to learn new things. So share what you're interested in getting out of this new opportunity! If you left involuntarily, explain why and be willing to admit fault. Show what you learned from the situation and how you want to prevent that issue from happening again.