You will often see two bigstacks calling the shortie's all in on the bubble with any cards, it's a sort of implied collusion and doesn't require them to know each other at all - it just makes sense to gang up on the shortie from an icm perspective. If that one guy goes home, everyone else gets paid. This is not 'cheating', it's just smart poker.
As for actual collusion - I don't know how good your grasp of ICM is, but DoN's are heavily affected by very small movements in chips from one player to another. The shortest stack on the bubble is a dangerous place to be, and even a small 2bbs chip dump from a colluding big stack can negatively affect the equity of every person at the table, as there's no longer an imminent payout for everyone else.
I don't think I explained that very well, sorry. But I've just looked on google and here's an article that may explain it a bit better:
http://www.daleroxxu.co.uk/2010/05/49-player-chinese-cheating-scandal-on.html