Seems like someone should take a look at the sites that fell/gained/fell. A third-party. Maybe an outside eye, looking at each of the sites in this thread that have ridden the down/up/down roller coaster could spot something.
I'm willing to be that third party if you'd like, but if I'm not suitable, then maybe another trusted forum member could be considered. Tedster, maybe? Someone willing to present honest thoughts and findings without tromping on the webmasters' feelings, while also keeping the URLs confidential if desired.
There's just bound to be some similarity (or similarities) that could tie these sites together.
Just a suggestion, but I think this one panda iteration might hold some answers, since there are a few that fit the down/up/down pattern so perfectly here. Might be the best time to find those answers.