It is no secret to those involved in SEO that Google has recently rolled-out or is testing some newly enhanced features (for usability and design) as well as tweaks to the ranking algorithm as of late.
The first wave felt weeks ago has now been dubbed “The Mayday Update”, you can also refer to the recent White Board Friday by SEOMoz for their .02 cents on this as well. To summarize the Mayday Update, many websites who had top ranking pages have found themselves on page 3, 4 and even 5 in many instances with no significant changes identified as the cause of why such a violent change in search engine result page position are clear.
Other side effects include that the long-tail and mid-tail keywords (phrases with 3 or more words) are passing through more stringent semantic and ranking filters (meaning the array of broad match keywords a page could potentially rank for) have been tightened up or reduced to increase relevance. Many have reported seeing up to 2/3 or their long-tail and mid tail traffic disappear during this transition.
For example, if I had a page that ranked for multiple keyword variations of a semantic theme such as kittens, kitty and cats and a corresponding modifier (like delicious) was present in tandem with such a query like kitty food, cat food and kitten food, those results are more than likely going to return a different set of results (as opposed to the same page) since the apparent PaIR algorithm is set closer to precise semantic sets.
If pages rank for fewer variations, then the broader less descriptive keywords in a query could be devalued as a relevant ranking signal on a signal to noise ratio (resulting in less varied, more specific results).
While in theory a slight change in the node of relevance a semantic phrase has when weighted with other ranking signals could send ripples through the web as queries no longer pack their flashlight-like breadth of scope and are now more laser-like in finding a corresponding set of documents.
It is uncertain if this directly correlates to (a) how data is being crawled (b) if the threshold of relevance is higher or more specific meaning the ranking metrics have been altered (c) if it is a filter of the repository or delivery from the Bin (the place where search results are pulled from) or (d) some facet of all of the above.
What is certain is that for many, their main keywords are in a blender with new competitors taking new positions as a result of the recent SERP (search engine result page) shakedown. Typically when things of this magnitude are rolled-out, tested or tweaks are in progress, it is not uncommon to see multiple data sets of the SERPs (liked cached copies or ghosts in the machine) of the various data centers fliupping back and forth.
When normalization occurs and the new algorithms settle, we may see many of the pages that were churned take their place back in their previous positions. However, in the meantime, the best thing to do is stick to SEO basics and keep producing relevant content, using intelligent internal linking to preferred landing pages using preferred anchor text and acquiring quality inbound links to offset any depreciation your website could be experiencing from the virtual rug being pulled out from the web as we knew it – as indexed by Google.