The Dimensional Shift was an event that quite literally changed the face of Dimension Number 2.
The Creator, who claimed by a certain point in DN2's life that he wished to make the world seem 'more original', actively and suddenly reset the entire dimensional specifications - that is, he completely changed the world without any warning.
The humans, not originally a part of DN2 or the Creator's creations, survived the shift, but ended up strewn over the new world, with no idea how they had suddenly had been forced into a new world.
It has been explained that with the Dimensional Shift, the citizens of the Dimension did survive, but were completely strewn about the new Dimesnion, their memories entirely replaced - although some, such as Meowth-senpai, seem to have kept their memories of the shift, it is completely unknown what has happened to most of the memories of the earlier companions of the humans.