For many people, this question is not an easy one to answer. You may have left on uncomfortable terms and the answer that comes to your mind may not reflect well on you. Your answer needs to be neutral about you, your former company, and your former manager. If you bad mouth the former company or manager it just reflects poorly on you. Keep your answer short so you can focus back on your skills and expertise that pertain to the position. Don't focus on whose decision it was to leave but what this current position gives you that your last one did not. For example, "I was not able to use my ... skills as fully as I would have liked" or "I am looking for a position that allows me to use my skills in....". You are not lying but focusing on going forward and not rehashing the past. Remember to look the interviewer directly in the eye when you say this so you don't appear to be uncomfortable with your answer or deceitful.
It's not where you've been but where you want to go that's important.