I have never heard of frogs flipping on their back until I worked at the zoo as an amphibian keeper. Each morning I would come in and find several frogs (Dusky gopher frogs [Rana sevosa]) on their backs. I would flip them around and they would resume normal behavior. None of the behavioral scientists at the zoo could come up with a reasonable explanation. Eventually, the frogs settled down and the problem went away. This type of behavior could be the result of the frog "playing dead" because it is frightened either real or perceived.

On the negative side, it could be a symptom of a neurological disorder caused by a virus or parasite. If this is the case, you will need to seek an exotic vet. As long as your frog continues normal behavior, I probably wouldn't worry to much. Unfortunately, if flipping is a symptom of a disease, there isn't much hope that your frog will survive.

I hope your frog gets better.