Well, normally, you need to tell people what they want to hear. For example, in an interview, HR people still ask the same questions used since two decades ago and are happy hearing the same standard replies, even if there isn't an iota of truth, as HR interview Q&As are easily available.
So, coming back to your question, the answer would be, "Since I was looking at growth opportunities where I could handle greater responsibilities somewhere down the line, I felt like I needed to explore new opportunities which would enrich me with new challenges and experiences. Nevertheless, I am looking for something that would help me progress in my career and be in sync with my profile at the same time."
The normal words people like to hear are "growth," "medical issue," "sickness & bed rest," "self-sabbatical," and "taking time out for sorting personal/family issues."
If you want to speak the truth (or maybe in a lighter vein), it would be as follows: "I had this Hitler kind of Indian HARI SADU type of a boss who never thinks beyond himself and believes he is the king, while I see myself as a poor peasant asking him for alms. So, one day, I decided to get rid of the daily begging for mercy and planted the seed of self-respect in me to grow. Since I could not kick him, the only option I had was to kick the job with him. Now, I want a better boss, one who has worked his way up and is a guide to me, someone who sees himself as an employee too. If you have such people, then be assured I would work for your company for sure."
Being an HR myself, I know one thing: make candidates undergo any tests (even a lie-detector polygraph). There is no hard and fast rule to select them, retain them, or remove them. Instead, have an informal relaxed discussion acknowledging that it's the need which meets the deed, and you can judge candidates better!
Regards