I can only speak about one leak I personally saw happen. Some guy calling himself Stuart Brawley (sp?), no idea if it was him of course, gave out "Xscape" to several fans back in the MJJForum days. At least in one case he supposedly traded the song for some nudes of a female fan. He sent it to me (statue of limitation is past right? lol) and some one in the MJJF leadership back then. And a few weeks after he/she started handing out the song, it exploded online and everyone had it.
The reason he gave me for sharing the track was that he was so upset about it not being included in Invincible and thinking that it needed to be heard by the world, because Michael loved that song.
But in general you know how it goes. Multitracks get released to people for remixing. Then someone gives it out to someone, accompanied by a dialogue like:
- But promise me, you can't show this to ANYONE.
- Totally.
- No, I mean it. Nobody. At all.
- Yeah, I get it. I swear, nobody is going to get this from me.
And then that person does the same and so on on and so forth.