In 30 May I disallowed a directory that contained only projects created by my site's users. Also, I set all user's folders to return a 404 error when accessed.
The next day GWT reported a drop in tracked pages from about 800 to about 50.
After 10 days I started noticing a moderate traffic/rank drop, and it's been getting worse since then.
What was responsible for the drop? The robots.txt policy or the 404 responses?