User blog comment:Zaarqawi/Why players from YGGDRASIL have been transported to the New World at various periods of time./@comment-27131588-20151108142213/@comment-27131588-20151108172728

And yet, this explanation has the most sense

Why the players were transported in different timelines? Because they were in different locations, or even worlds. Why they were transported in groups? Because they were in same zones/guild houses/etc.Why 8GK were transported with their guild house? Because they were inside it during the end of game, just like Momonga was.

Though, we don;t certainly know whether only the NPC/Guild Houses/Monsters which were close enough to players (or were in the same zone) were transported to NW