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