aakk9999 - 3:02 pm on Aug 10, 2013 (gmt 0)
A few things I can think about:
1) You may be affected by Google's Rank Modifying Patent [webmasterworld.com...]
2) If the ranking of your pages is influenced by user metrics then a change to your page may have reset those and Google will have to build these again.
3) The new content added has changed the focus of your page so it does not rank well for phrases it used to rank, or the page title is not so relevant to the page any more
How long ago have you made the change?
How long after the change did the page dropped?
How much time has now passed after the drop?
Maybe you just need to wait it out a bit longer for the ranking to stabilise?