I'm not sure what it it is about late summer, but I have noticed the last few years that my spam is at its worst in late July through September and then it tapers off again until the next summer. I have no idea what the correlation is, but it has been consistent the last three years.
I don't worry about it, however, because my Inbox stays pristine with the help of Cloudmark Desktop. You have heard Adam and I rave about this product for nearly four years now. Without it, I would have probably given up email years ago!