The whole situation goes to show that you can’t silo your SEO success and failures because there are so many working components going on at the same time that could have an effect. For instance, a strong PPC campaign might be driving 10-20% of your traffic over the course of several months. When you decide to cut the budget on that PPC campaign (or pull the plug entirely) that 10-20% of traffic is going to get cut back with it.