I read that the recent Google "Mayday" update had affected a lot of e-commerce sites.
Long tail keywords such as those commonly associated with product names were affected. I found the following from a Webpronews article quoting an ex-Google worker.
t"his is an algorithmic change in Google, looking for higher quality sites to surface for long tail queries."
"This change seems to have primarily impacted very large sites with 'item' pages that don't have many individual links into them, might be several clicks from the home page, and may not have substantial unique and value-added content on them,"
"For instance, ecommerce sites often have this structure. The individual product pages are unlikely to attract external links and the majority of the content may be imported from a manufacturer database. Of course, as with any change that results in a traffic hit for some sites, other sites experience the opposite."