It's a black box from an outside perspective, but generally I imagine it's factors such as appearing political or the sort of thing that would be reported in mainstream media (both of which are somewhat discouraged by site guidelines), or being repetitive (there have been a number of articles discussed recently on this topic).
This is not a new issue. Here's a discussion on the subject:
This is not a new issue. Here's a discussion on the subject:
https://news.ycombinator.com/item?id=13857086