Panthro - 4:50 pm on Aug 1, 2012 (gmt 0)
Would be hard to tell without a closer look, but maybe your site is "over-optimized" in that it just has too many links.
Of course, if I remember correctly, that didn't seem to start affecting people until Penguin came along. If your traffic dropped in March, could be something else.
Of course, sometimes it can be hard to isolate any one particular cause.