Have you identified the kind of environment where you do your best and are happiest on the job??
I would simply say although you learned a lot at that job, you felt you and the position were no longer a good fit; you can say you've evolved, grown... have new challenges you wish to explore, etc. You might also say that due to this you felt you weren't doing your best work. If they ask for clarification, you have a chance to steer the interview towards what kind of workplace bring out the best in you, and get some insight about their workplace conditions.
Although this approach hasn't always gotten me every job offer I interviewed for, it HAS weeded out positions where I knew I would not be happy. Remember you are interviewing THEM too and looking for that 'good fit' for you..
Good luck. I'm a firm believer that just because somebody loses a job, this needn't automatically reflect poorly on them. We have to be honest with ourselves about what happened in the failed job, though, and do some soulsearching to find out our role in it so we can move on and make a better decision in future jobs.