Because of the algorithm change called “DMCA Penalty”, which Google announced on 08/10/2012 and implemented a week thereafter, websites for which Google repeatedly received DMCA takedown requests are now punished, which puts them in the back of the search results.
As a signal for a possible penalty, Google looks at the number of so called “valid copyright removal notices”.
In other words: Google punishes websites that have many “DMCA takedown requests” and thus are suspected to go against copyright law.
What is a DMCA takedown request?
The abbreviation “DMCA” means “Digital Millennium Copyright Act”. This law from the United States of America extends the rights of copyright owners.
A so called “DMCA takedown request” is a request from the copyright owners or governments towards Google to remove a page from the search results or the index, based on the DMCA.
If a website was removed with regards to a US Digital Millennium Copyright Act complaint, a hint will appear on the specific search result page:
Which websites are affected?
Google releases the number of requests received from copyright owners and governments on a website specifically created for this purpose and provides a list with domains to be removed (see link below). This way you can get an idea of what “kind” of pages are affected.