I'm in this situation right now. I have a rough sketch of something I might say, depending on the employer and interviewer's personality. Any input on this would be appreciated!
Department director, who I was working closely with, and who delegated me many responsibilities, left and it caused department upheaval and reconfiguration, leaving me without my beloved patron who I was a loyal subordinate to, and causing chaos in my working environment
A few other co-workers left a few months before without anything lined up, and found good jobs in only a month or two, and recruiters were hitting me up regularly, so I figured it was something I could pull off
I had some heavy family stuff happen that I didn't take an opportunity to properly mourn, and I also wanted to reevaluate my career direction; take an opportunity to refresh myself to be better at my next job
Saved up a year and half's worth of expenses so quitting was a carefully rationalized decision
And a few other things
The truth of what happened is that my department director (a woman, I'm a man), started coming on to me, like, really aggressively and weird, and I threatened to sue. She quit and found a new job, particularly since she was getting heat for running it into the ground.
I quit because the whole situation fucked me up pretty bad and I needed to recover. Of course I'm not going to say a word of this to any hiring manager, because I'd be insane otherwise.
I'm going to try and make it an opportunity to show that I uphold the bureaucratic values of loyalty, being an effective subordinate who management likes, providing strong work that makes the people above me look good, rational risk taking, career-focused entrepreneurial spirit, blah blah blah.
Does this sound good to any of you? I'm still working this one out.