Now let me preface this by saying i have rather spotty knowledge of the lore of WoW, I know about certain things here or there, but i dont know everything, so I apologise if i have misunderstood something.
However, isnt there a chance that the wound in silithus has pierced C'thun's prison? as far as i was aware we may have fought him back in classic, but he was very much still alive, and with the appearance of Azerite over Azeroth now, I would assume that would imply somewhat that Sargeras managed to at least reach or damage the Azeroth world-soul with his final strike, If C'thun is indeed alive would that not now give him a clear path, or at least a path of much less resistance to reach Azeroth's world-soul?
We have had Azeroth's world soul empowered denizens of the planet before, a'la the Well of Eternity, but never has it seemed to have an immediate effect on the personalities of the people it has come into contact with, watching the Horde ending cinematic, and what i have read about the Alliance cinematic (Havent watched that one just yet) It seems that Anduin has a noticable change in his personality upon coming into contact with the Azerite, and it almost seems as if Sylvanas does as well
Perhaps that also has something to do with the imminent faction war we are about to experience, I mean after literally fighting off a massive, planet-destroying legion of demons and their master by working together, you would imagine at the very least there would be a somewhat respectful, 'live and let live' attitude between the factions for at least a little while
Could it be that the wound in Silithus has broken C'thun free, and he is starting to infect the Azerite bubbling to the surface?