On the crest of a wave

One of the things I've noticed over the past few years is that spam seems to have some form of cycle when it comes to its prevalence.

Right now, my guess is that we've just passed something of a trough in the cycle and it's heading for the crest again. I base this on the number of mails trapped in my ISP's spam filter - a week or so back, it was quite low, only 60 or 70 over a 24 hour period. However, we're on the way back up now, to around 100 per day. At it's peak, I'd guess the deluge amounts to between 160 and 180 - it's been a month or two since I saw that, but it definitely took a dive from there and is only now recovering. This pattern isn't new - I've observed it over quite some time.

Why this might be is something that puzzles me. It's been fairly well established that the majority of spam comes from relatively few players in the field, but what affects them all simultaneously to give rise to this cyclical effect? We can disregard issues such as the US' CAN-SPAM act, which never seemed to affect the quantity of junk email one bit., or the hurricane season (takes out all those Florida-based spammers temporarily), because these haven't been functional over the past few months. But something out there is having a depressive effect on spammers' activities, and it's acting across the field. Now, if I could only catch whatever it is, bottle it and label it, I might be able to sell it - probably via email marketing.