Think of it as Page Rank in reverse or Negative Page Rank.
It used to be on my site if I had PR5 on the home page, a page linked from it had PR4. A page one directory level down from that had PR3, etc., etc., and so on and so forth.
What if Google now applies the same PR concept it had applied for strong pages for weak pages?
I used to get ranking simply from anchor text in internal links.
What if the same type of idea is now applied negatively. The more internal anchor text links to bad pages, the more likely, Google estimates, users will have a bad experience on that site.
In theory then you have to remove the weak pages and/or the links to those weak pages.
Google used to lower rankings for sites that linked to "bad neighborhoods" on other sites. What if Google now lowers rankings for links to weak neighborhoods (pages) on your site?
As much as I hate the effect, if I understand the theory correctly, it's very clean, logical, and consistent with the original Google principles.