If you have wondered why your site traffic dropped considerably from more than 30% to 40%, it’s because of a change in May Day Update in Google Algorithm for Long Tail Searches. Long Tail Searches are those small number of hits that pages of your site get and don’t get noticed unless you drill down to find them. For example, 100’s of your site pages might be getting visitors for search terms that are significantly less important.
Though according to the Pareto Rule, 80% of your traffic is brought by 20% of your article, I guess the rest of the article (80% ) might be getting you good traffic, and it’s important too. Since there is nothing you can do about it, you might try to update your articles and see if you can add more value to them, and maybe (my assumption) you can get traffic back.
Matt points out that these updates are permanent and have not been done manually but as said, changes in the Algorithm of Google Search bring this effect. These searches were rolled out between April 28th to May 3rd.