I don't see a recovery. I think in my case it has to do with sitewide links from authority sites. I always had a huge number of nofollow sitewide links from a few authority sites. It seems to me the links don't pass on the same authority as before. As posted above most nofollow links disappeared in Bing WMT suddenly.
Lesson learned, never stop link building and never build more than 1-5% of nofollow links for a healthy link profile. I'm curious how long it will take to get the 60% of lost URLs back in their index after gaining some new authority dofollow links.
I think "static rank" should not be a query independent value. I don't get it why they remove URLs with no negative feedback from users just because they don't have the same link power as before.