I was assigned a Project for SEOizing somewhere in July.. When I assesed this site's source code.. I found keywords stuffed in Comment tags, For eg; <!-- Keyword, Keyword1, Keyword2,..... and so on --> As far as I know, this is supposed to be Illegal... So, I eliminated all these Comment tags from the source to avoid getting Banned in SEs. Soon after I did this... The site traffic started to go down... The SE qeuries are still as highly targetted, but the traffic reduced considerably... More than 50 % reduction. DO u think... I should reposition those Comment tags????
It could be a factor of localization of results, or it could be the fact that your keywords have dropped in popularity. Rarely when I see posts like this does one have the ability to point to a single factor that affects your ranking for a particular keyword. There are a lot of factors within your control that could also have been changed, and a lot of factors outside your control that potentially have been modified. Projects to SEO an existing site with acceptable traffic levels have been known to backfire on occasion as well - overoptomization could certainly have triggered filters, as could an abnormal pattern of inbound link growth. Only in a controlled sandbox can you really begin to estimate the effect of certain techniques and even in controlled sandboxes things happen - algo's change over time, they seem to differ at least partially based on keyword, and then there's the quality changing factor for the inbound links - what shows up on the toolbar as PR5 may not be exactly what the page is considered internally by the SE's. Personally, I've seen keywords in comments have a positive affect, but then again I've never actually tried to just throw in some keyworded comments all by themselves, and it's not something that I would generally recommend as it just tends to be over-doing it. Your best bet is to do a thorough analysis of top ranking sites and emulate them to whatever degree that you can - on page SEO is one thing, but obtaining high quality links is another all together. Another top of the list thing for me is internal link structure.
Analyse what search terms used to send traffic to your site and compare it to the current stats. Once you figure out where the rankings have gone you can analyse what has caused the drop.