Purely speculation, but an 80% loss of google referrals is pretty extreme and (to me) screams of the result of some kind of algo based filter, if not a manual interaction.
This is one of the times when Google could really gain some webmaster respect by actually sharing "WHY", or at least giving you a specific pool of reasons why your established site tanked.
Gaining authority and getting site-links could easily mean that you rank much better for some commercial keywords, thus triggering an automatic flag and manual review for site quality.
Again... all pure speculation, and in the "what would I do" realm of actions, very little to nothing! Aside from making sure the site is not "Adsense or Ad-heavy"... and since its WordPress, "Make Sure it loads very quickly"... I would just sit on for 30-60 days and see what happens.
If you MUST do something to please your own curiosity or need to generate revenue again, work on Facebook, Twitter, Local, Niche forums, ANYTHING to generate an alternate steady stream of traffic, and let Google settle first.