End of fiscal year. If you're attracting ads from companies that do serious advertising, you may find a highly predictable once/year drop in sales. A couple of big advertisers get within a couple of months of the end, see that sales predictions have not measured up, and cut the advertising budget to help soften the blow for the final report.
Industry-wide belt tightening. Most of your revenue comes from ads for widgets. Newspaper headline blares "EU puts a tariff on widgets.". Advertisers pull back on their ads.
Heh, that's what more than one person in this forum said before discovering that they had made a site-wide change that screwed up their display of ads.
If that really happens, watch out -- WebmasterWorld will need an additional forum just to contain all the new weeping and moaning threads! Alas, I can't find any announcement of this online.