I believe it is the media scanning software that YouTube uses. It scans the audio from the videos and then searches through its library of mainstream music and when it finds a match in the audio waveforms it alerts YouTube that infringement is occurring. Facebook recently starting doing the same thing. Once a mainstream song is released I believe it is put into some sort of archive and held on record and this audio scanning software can detect duplicate versions and removes the videos that aren't authorized to be put on there.
There are a few loopholes to this though. If you record a video and edit it with the audio actually being part of the recording then you can sometimes pass through the software without being flagged due to the white noise which is picked up by your videocamera. When you take the mp3 of a song and try to add it over a video to make the song crisp it gets detected and is flagged.
I messed around with this a lot back when I was getting videos flagged that had music overlayed in them.
After a video is flagged the notification they give you referring to Strange Music Inc. or Universal or Def Jam or whoever isn't the company that is directly making you remove your video. It is the company that the audio sweeper detects as owning the rights to the song YouTube then makes the decision to either warn you, remove the video, and/or remove your account because they see that the sweep shows your uploads as infringement. So when they send you the message about deleting your account all they are doing is referencing who owns the content you are posting.
I hope this clears some things up for you.
There are a few loopholes to this though. If you record a video and edit it with the audio actually being part of the recording then you can sometimes pass through the software without being flagged due to the white noise which is picked up by your videocamera. When you take the mp3 of a song and try to add it over a video to make the song crisp it gets detected and is flagged.
I messed around with this a lot back when I was getting videos flagged that had music overlayed in them.
After a video is flagged the notification they give you referring to Strange Music Inc. or Universal or Def Jam or whoever isn't the company that is directly making you remove your video. It is the company that the audio sweeper detects as owning the rights to the song YouTube then makes the decision to either warn you, remove the video, and/or remove your account because they see that the sweep shows your uploads as infringement. So when they send you the message about deleting your account all they are doing is referencing who owns the content you are posting.
I hope this clears some things up for you.