Google is claiming that ISPs in Turkey are intercepting their DNS service. The implication, unstated in Google's announcement, is that this is being done in order to block services such as YouTube and Twitter which are banned by the government. The company cites "credible reports" and their own research.
Turkish authorities ordered YouTube shut down, just as they had ordered Twitter shut some time ago. The order given to ISPs to remove access to the services in their DNS, but many have been getting around the problem by setting their DNS to Google's free public DNS service (primary: 184.108.40.206, secondary 220.127.116.11).
Google is claiming that Turkish ISPs, apparently under order of the government, are intercepting access of Google's servers and redirecting them to their own DNS, thus re-enabling the ban.