Here's what I've seen and maybe someone can agree or call me crazy:
Search suggestions. You know the suggestions that drop out live while you're typing at Google and Bing? They change from time to time and MANY users take the suggestion, effectivly, the searchbox is redirecting traffic.
We have a page, lets say "trouble-with-dirty-footwear.html" using the terms for all kinds of footwear and how to clean them; one week it's flooded with traffic for "trouble with dirty shoes" (poor ad clicks) the next week it's getting the same traffic for the term "touble with dirty sneakers" (high click throughs) but no traffic for the previous term. We rank equally high if you check SERPs for both at the same time but people aren't being directed to that search because the searchbox completed it for them.
To test this we manually check the suggestions by typing "trouble with dirty" and the term that's popular at the time will complete the search phrase and that's how I confirm why we get "sneaker" traffic rather than "shoe" traffic. It's hard to guess the actual percentage the effect has but on the few that we can it's about a 75% shift from one to the other, enough to make you think you lost rank.