sorry to hear that, same #*$! happened to 2 of my sites. Luckily i have bunch of them spread-ed across different webmasters/adsense accounts so i dont really mind to loose few of them to learn something else and valuable.
so, what exactly happened to me? Well, very same #*$!. Almost all keywords got hit and went from 1-5 to 25-30. Traffic (one site) from 6k to 800 unique (still decreasing), adsense for that site from XX to X EU. Last 3-4 days were horrible, yet again i don't give a *** as still got many webs and still make killing.
Issue: Still speculating (as you do), yet most likely it was hit due to outbound links (i used cracked WP theme on my sites and some backdoors were left on themes i use, so some smart ass 2.5 month ago left a bunch of sex related outbound links), made them visible only for robots (google bots).
I did receive a message to my webmasters account saying exact thing "i was hacked and my content was cloaked". so this might be an issue to me. I cleaned everything up and waiting to get my rankings back, if not i just throw websites in the trash and will keep working on another webs.
I advice you to inspect all of your outbound links (there are bunch of tools both paid and free to do so)
good luck pal towards recovery.