How do I address the question, Why did you leave your previous job?

On April 17th I had a meeting with our office manager. At that time I was presented with new job expectation I didn’t feel comfortable with. I felt these new job expectation fit my skill set, and were achievable long-term, but not in the time frame they were expecting, and were unrealistic to accomplish with my other duties. More to the point, these expectations were never discussed before, and were sprung on me. What was expected of me varied from day to day from acting as a consultant for the company to being a traditional employee. I am not afraid of a challenge, but what was presented was not possible. I explained this to the office manager. By the end of the meeting I had every reason to fear losing my job, if not immediately, in a short time to come. The next day the office manager requested another meeting where he proceeded to apologize for the way he presented these new expectations (in an extremely emotional capacity), but gave no real solution or clear direction for moving forward, which left me in an unsettling position. Over the course of the next week there were multiple, uncomfortable, positions I was put in regarding their personal and professional problems with another employee, the only other employee in the office. Overall, I was constantly faced with emotional responses on behalf of my employer to legitimate employee concerns I had, which made for an uncomfortable professional setting. After a final, uncomfortable, situation I was faced with regarding their personal and professional concerns with the other employee, I no longer felt comfortable returning to work. I sent a letter of resignation and received no response regarding my offer for two weeks notice or an exit interview. They also changed my password to my work email which indicates they have no intentions of having me back in. I am now concerned how I move forward presenting this situation to a potential future employer. It is important to note that our office consisted of two employees, including myself, and two bosses, 4 people total involved with the entire company. The office manager I mentioned was also one of my bosses, so there wasn’t anyone other than them to bring employee concerns to. The job I will be interviewing for is in a corporate setting, as opposed to the small business setting I was in. One of the reasons I am interested in this positions is to hopefully avoid situations such as the ones I faced on my continued career path. How do I address the question, "Why did you leave your previous job?" How do I address the fact that I wouldn't feel comfortable using my previous employer as a reference?