This was ages ago and I don't know if the people behind it did it on purpose. In hindsight, it must have been the case. Basically they created a website where they shared (their own?) "copyright free" music, which was licensed with CC BY. The music was half decent and just some instrumental EDM / DnB / Jazz. In 2012-2014 there wasn't much decent copyright free music available, so they had little to no competition. Everyone who hopped on the YouTube money craze was looking for some copyright free music, which they could use on their monetized videos. So the demand was definitely there.
Fast forward 1 year or so, suddenly most if not all of the music tracks have content ID and full copyright enabled. So all the people who created countless monetized videos with these "copyright free" songs basically got tricked and owed any further revenue from those videos to the newly assigned content ID holders. If you want to do some digging, here's one Artist/Label who did this. I know it because I've used a song from them, which was advertised as copyright free back then:
Artist: The Passion Hifi
Licensed to YouTube by: Interstreet Recordings; Audiam Canada, UMPG Publishing, and 6 Music Rights Societies