Google lately noticed that publishers cleverly tricked the Google search ecosystem by putting thousands of freelancers to work to generate pieces of low-quality content written around stuffed keywords in order to gain search prominence for varied search terms. The blog noted : https://www.brainpulse.com/articles/revisiting-google-panda-update.php
Google Panda is a change to Google's search results ranking algorithm that was first released in February 2011. The change aimed to lower the rank of "low-quality sites" or "thin sites", in particular "content farms", and return higher-quality sites near the top of the search results.
The stated purpose of the Google Panda algorithm update was to reward high-quality websites and diminish the presence of low-quality websites in Google's organic search engine results. ... We've tracked 28 data updates to Panda between 2011 and 2015.