Mostly because of what has happened in BFA.
-Someone whispered to Voljin to make Sylvanas warchief. We don't know who did this.
-When Bwonsamdi lost Voljons spirit, he said "Da boss ain't gonna like dis", meaning he answers to someone else, some mysterious Death related entity.
-At the end of 8.2.5, Loyalists to Sylvanas get some extra info from her, with lines like "Countless souls h ave been fed to the hungering darkness," " they stubbornly clung to hope. To life. They will learn the truth, along with all the rest," and "The armies of Azeroth will fight her master, and he will line their streets with corpses. In the end, he too will serve Death." She clearly is serving Death.
-Ilgynoth has a new line in 8.3 that states "Before the last shadow falls, the Father of Sleep shall savor his feast." Father of Sleep is word for word one of Mueh'zalas names, in addition God of Death, Son of Time, the Night's Friend. "savor his feast" links to Sylvanas feeding the hungering darkness. Mueh'zala is also described as ever shifting form and huge shadow.
-"Together, Mueh'zala and the dreadlord said that there would be a reckoning, the battle would come, the fire would burn, and Mueh'zala would feast on all of Azeroth" from Warcraft short story book.
-It stands to reason that Mueh'zala whispered to Vol'jin, as a Loa(which he supposedly is, so Vol'jin would listen. Sylvanas becoming Warchief then serves Mueh'zala, allowing him to feast on the souls lost to war. It appears Sylvanas has been in the loop about this, I'm guessing going back to her deal with Helya. Potentially giving her soul and fealty to Helya's boss, whom is also likely Mueh'zala.
-Helya's boss likely bein Mueh'zala is referencing back to when Odyn made a deal with "Death" and gave one of his eyes to see into the Shadowlands. He described Death as an ever shifting shadow or something along those lines.
I'm sure there are a ton of other threads pointing to him, but these are the most obvious. It's pretty clear he is going to be a thing.