Google May Day Algorithm Change
A major change in Google search algorithm took place in May 2010. The algorithm has been tweaked for long tail queries such that higher quality sites appear in SERP.
Individual long tail queries would not provide much traffic, but in aggregate can provide a significant amount of traffic. Large sites with product pages that would not have individual links into them, be several clicks from the home page or be hidden behind search forms (dynamic pages), and may not have unique and value-added content on them would be impacted. Smaller sites with good or even better unique content, but with few incoming links, and attracting little traffic should be benefited.