User blog comment:Grimraven/Players or Suspected Players in the New World./@comment-27023992-20151002224552/@comment-5480911-20151006143204

Well there are theories....The closest we have to confirmation is that one....they show for a period of every 100 years too...it involves some kind of Dimensional tear..as when Zy'tl Q'ae was shown to have appeared falling torugh some kind of opening in the sky as quated by Pinision.

Since they appear at a fixed time...that implies design....which means either someone is deciding to bring these players every 100 years trough some kind of magic or instrument....or it could be a natural phemonana that involves certain elements and conditions that form exactly 100 years or so.

Either way later down the road we should get more clues and evidance as to what is bringing these players here and what is the connection between them.

SAO involved a madman trapping players in a game world

Log Horizon seems to indicate it was the involvement of an alien race from another universe.

No Game No Life involved a God.

So any of these theroies or even other circumstances can lead to an explanation..so we just have to wait and see.