The earliest it could have happened is during Wrath of the Lich King, as we know people who died before then ended up in their appropriate places.
And the most likely cause is someone or something very powerful entering the Maw and turning it into a kind of black hole. Given Sylvanas' involvement I would guess it has something to do with Arthas and/or Ner'zhul.
Remember AU Ner'zhul's last line when he died in WoD (and in the Shadowlands no less) - "My power will ripple across eternity".
I'm thinking that when Arthas died Ner'zhul piggy-backed on his soul and entered the shadowlands with him. I can even see Blizzard retconning this into being his plan all along.