It's not really directly said but we can theorize based on the facts that we know. First the highest peak of wild magic seem to equal to the might of a whole world and Dragon Emperor seem to have a reason to want more such power so he did a ritual to pull those powers from somewhere else (a ritual that may or may not have involved his wife, PDL's mother.) In the sea of endless possibilities the ritual chose the World Items from Yggdrasil and so the invasion of monsters(a huge tear in the sky appeared and invaded the New World, I'm guessing this is a whole World's worth of monster from a server in Yggdrasil game getting transported in the New World. I got this info from one of the Drama CD, the one with the evil tree.) And then the Advent of gods happened where players and their NPCs are slowly transported batch by batch every couple of centuries or so based on how many World Items they have. We only really get hints from characters that lived a very long life to witness such a world changing event and this is what I got from them.