Keep an eye on her. Sometimes if the bedding is deep, they could get stuck on their back. Especially if you have a brooder heat plate, like the ones from Brinsea or Premier, where they could get on top of it and take a spill off of it.
If you keep seeing this one on its back and struggling, then I would get worried that it might be another cause. Then it might indicate there's something going on neurologically. This is not meant to scare you, but I had one duckling at 5 weeks get stuck on her back and struggle to right herself. After the first occasion, I too thought it was just an accident. It kept happening though, and the vet said it was seizures.
If you keep seeing this one on its back and struggling, then I would get worried that it might be another cause. Then it might indicate there's something going on neurologically. This is not meant to scare you, but I had one duckling at 5 weeks get stuck on her back and struggle to right herself. After the first occasion, I too thought it was just an accident. It kept happening though, and the vet said it was seizures.