Well, just from getting into search engine optimization and learning about both white and black hat methods, I know that some people can attempt to knock you out of the SERPs by running bots designed to eat up your bandwidth. Once your page is shut down for several days to a week, the search engines drop your rankings.
I have absolutely no idea how to verify this, or how to do anything about it if this is indeed the case, but just wanted to throw that possibility out there…