It's not a coincidence. But it's unlikely Google is manually penalizing number one ranked sites. I believe there are several things going on.
Firstly is that the factors leading to a number one ranking can change. Rankings don't mean that the first listing has the best ranking factors of all other sites. It just means they have the best ranking factors for the moment, for that niche, AND for what Google thinks is the user intent of that phrase. Refine one of those factors to try to make the results better, or just throw in a random factor, and if the ranking factors supporting the current number one site aren't broad based enough then the rankings are going to change. By broad based I mean that they have a wide range of ranking factors in their favor, not just 87 out 200 (that's a simplification for the purpose of example).
For a few months the factors may favor a site with a certain kind of overall profile of links and content. Then the next month there may be other factors added in to favor a different overall profile. I see this somewhat frequent re-ordering for various phrases. It's usually the same six sites trading places.
Another issue that I believe may have an impact is user satisfaction monitoring. That if a site experiences a certain amount of click-backs to the SERPs or if other metrics indicate satisfaction, that a site might earn a higher or lower place in the SERPs. Bill Slawski posted about a related Google patent last year [seobythesea.com].
I am confident there are other factors in play as well. Hopefully someone else will add to the list.