Frog flipping seems to be a common occurrence. Working as an amphibian keeper at the zoo, I found many of the frogs and toads flipped on their back appearing to be dead. I just flipped them back over and all was well. There are a couple of theories (other than an accident): stress (real or imagined; maybe an "unken reflex"), a neurological disease (severe motor seizures) or dietary deficiency. You may want to offer calcium dipped food items - no more than twice a week. Most likely this was just a freak accident.