How to explain why you left your last job for essentially the same job?

I worked at a small firm for a year before quitting. With different roles I've been able to use the whole "exploring new opportunities/challenges" reason, but I am unsure how to answer this question when its the same role. Obviously I don't want to trash my last employer and I didn't leave on good terms.

Any advice is appreciated.

usa jobs
usa jobs resume
usa hotel jobs
usajobs
usa jobs federal government
usa job in ksa
usa jobs
usa jobs login
usa jobs gov
usajobs.gov
www.usajobs.gov
usajobs.com
usajobs
usajobs.gov official