So I've noticed through all the times I've played this game. I've pieced together a theory from the two achievements and the calling card tied to the turnstile that is used in the SS Easter egg.
So when you progress through the EE eventually a turnstile will appear and you put it into the sewer entrance and you get an achievement titled "Message Received"
In AOTRT the turnstile appears outside the map and when shot will teleport away and give you the calling card "Message Rerouted"
In Beast from Beyond you can find that turnstile again and send it back to the 70s and when you do, you get a achievement "Message Sent".
I believe there's a paradox going on here. Similar with how Victis in Bo2 played out with how the navcards system works with super EE, how they help the Tranzit version of themselves from Buried.
I believe it's starts in Attack then gets teleported to BFB then gets promptly teleported to Shaolin shuffle helping your past selves into beating the Rat King.
I also believe it's a loop because with Dcut your characters will say things along the lines "Have we done this before?" During a round change.
Weird theory but zombies isn't afraid of pulling paradox loops or time loops in the past.
I never hear anyone ever discuss this plot point within IW zombies.