All to often whenever there is a sudden drop in Google traffic people start looking for issues on their own site that could have caused the drop in ranking. Sometimes its exactly the opposite. It may be things other sites have done to increase their ranking, and as a result your ranking has dropped.
There are a lot of other factors to take into account, for example has your site had any recent downtime?
On your own site. Have you made any changes in the past month or so?