mygamecube
Smash Lord
While it's not fact, this is the most probable assumption as to what happens. I'm going off what the past Smash Bros. games have done. That is not a crime good sir.It doesn't say that anywhere. Don't make assumptions sound like facts.
The other possibilities, is that it's completely random, which can also tie into the holding of L and R theory. One thing we now know for sure, is that Mac does not have to be present for the stage to change.