I have seen frogs flipping on their backs at the zoo I worked at. Most of the amphibian keepers agreed that this is a fear response, especially by newly obtained animals. Frogs will often flip on their backs because they recognize danger, either real or perceived (could it be the unken reflex?) It's been my experience that this behavior will stop when the frog becomes comfortable with its surroundings.

A more scarier explanation is that chytrid may cause a neurological condition called "absent righting reflex" (inability to right itself). If you frog appears healthy and feeding well, I am reasonably sure it is a fear response. Keep close watch to make sure no other symptoms develop.