My toads, when really small, if turned onto their backs would do the same thing. I called it "Playing dead toad". I always just chalked it up to a defense mechanism. Once they felt safe and realized there was no danger in their environment, they flipped back over. My assumption is that it an automatic response to a fall in nature. If they fall in nature, they have no idea what they are falling into so their best chance for survival is to play dead until they assess the situation. My toads are all wild caught and as they get older and more comfortable with me, they no longer do it.