I chatted with a rep from Sony, but she didn't seem to have a good answer. She wanted to reformat my Sony reader, but I didn't do so because she couldn't answer why the book opened an hour later.
She did give me this link about DRM bugs, but I don't think it was applicable in my case as all my books are in epub format.
http://www.kb.sony.com/selfservice/d...nalId=C1008686