It could be a number of things. New advertisers targeting the same keywords, decreased quality scores, combinations of both.
Many accounts follow the 80/20 rule where 20% of their keywords pull in 80% of the accounts traffic. If something happens in that 20% category, the whole account will start going down. Take a look at your 20% and you will likely see some signs to what is causing the drop.
All the best, Robert |