I worked as a software engineer in a big internet company for 7 years in total. For the first 6 years, I had been working at the department of desktop applications. I really loved working there. Really enjoyed the technology and the people in the department.
Unfortunately the department was closed a year ago. During that time, my company bought a small start-up that specialized in mobile applications. Upper management decided that I should be assigned to work in that start-up as a part of the team. I had never worked before with mobile and it was totally new to me.
After two/three months, I realized that I wasn't interested/content with that job. In addition, I was being constantly micromanaged by the new boss. Throughout that time, I wanted to find a new job but the current gig depleted most of my energy and my time. Also, due to personal circumstances(mortgage and etc.), I didn't quit and continued to work for a year in total. In the beginning of May, I got fired.
I know that similar questions were already asked here but I think every situation might be different.
Question: How should I answer the why did I left my last company after 7 years of work?