I found absolutely nothing unsual - I checked everything twice as for example GWT. As already said the pages are online since 2004 and just small changes were made a few times. No changes were made the last months.
I believe that the drop is related to the story "Google To Target Overly SEO'ed Sites Within Weeks" which was published on 03/16 (the first drop was 03/22). Moreover, I now have an idea what caused the problem. I just fixed it and now I'm waiting if the pages are recovering or not. If they reappear in the index I'll let you know.