One culprit could be wrong canonicals. Last Thursday they made a change in their CMS and uploaded a wrong canonical URL for all pages.
They're going to fix that today. Hopefully it will solve some of the problems
I'm not totally sure if the canonical really caused the drop, or just the final nail in the coffin, because:
- the wrong canonical was placed throughout the whole site, yet some pages did not disappear (e.g. homepage, pages of non-experts which have less authority than their experts pages)
- the wrong canonical was placed Thurs. but keywords started disappearing Tuesday.