Mine happens even with a high charge on the battery. Mine happens whether the book is on a card or the cybook's memory. I do not sync with the desktop reader. These may be the explanation for some, but not all. There seems to be many reasons why this happens because many people describe different symptoms.
It happens to me every time I've been actively reading for a couple of hours or more, which is only a portion of my reading activity. It also happens sometimes when I've only been reading a short time. I try to remember to reboot every hour or so to prevent it from happening, it's a pain in the butt to have to go searching for your place.