Copyright owners can use a system called Content ID to easily identify and manage their content on YouTube. Videos uploaded to YouTube (private or public) are scanned against a database of files that have been submitted to us by content owners. More info: https://goo.gl/Vf3DJN
Any upload that contains modified version or the original content itself, owned by someone else, could end up with a Content ID claim. These claims are issued by companies that own music, movies, or other copyright-protected material.