Well, having now watched the fourth Mayhem, I have to say: it's such bull****.

I don't even believe it's staged by the players, I think the DM just outlandishly decides one side is going to win and grossly advantage the side he favors.

It's four times in four fights that it has turned into player vs player + DM. Arguably 3 times if you consider that the Mind Flayer in the second match attacked everyone.