That is a significant drop.
There weren't any official updates in May, but Google continues to target guest blogs. They appear to be picking those off one at a time. You can be a victim of this either by being a guest blogger who points links to your site, or by allowing guest bloggers to post on your site.
If you saw a simultaneous drop in search engine traffic from Google, Yahoo and Bing, the problem moist likely resides with your web site or hosting company. Due to repeated attacks on servers, some hosting companies are restricting access to their servers.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin