It's buried in the massive "scammers" thread on kboards.
To sum it up, Amazon removed a number of books for a short period, and then restored them with a punishment that is, I believe, new. The books no longer have a rank.
Some authors who claim to never have used bots were also caught up in this. There is some disagreement about what else may have been the cause of rank stripping, from a technical glitch, to the practice of "book stuffing" (arguably, a method of generating unearned KU page reads, and/or creating a poor customer experience), to including incentivized links to the back of the book (a practice Amazon has explicitly deemed unacceptable as it generates unearned KU page reads).