It does happen at a lot of times that you believe your website is quite well optimized for search engines and you see that in the results about the traffic diverted to your website, but you see a sudden drop in the rankings. Completely oblivious to the fact that it might be due to over optimization, you start searching the key components of the website:
1) The contents
2) The outbound links – if not junk and obsolete now
3) The Inbound links – if not coming from just any unwanted website
4) Insufficient or too many keywords