Before I start, I’d like to warn that this speculations will cover spoilers both from diablo IV and Wow : Shadowlands. So you’ve been warned.
TLDR : We are like the Nephalem in diablo, a unknown variable in the cosmic destiny, and by defeating the legion we may have broke the balance on a cosmic scale and disturbed the Arbiter’s Purpose, making her goes offline as it was not foreseen by any cosmic forces in the universe. Explanations in the TL post following :
First of all, its common knowledge that blizzard recycles some plots from its different games , and even use the same plots in differents expansions ( wink @ MOP && BFA, world tree, world stone etc … )
So when blizzard said " We broke the machine of death when we did something in Legion " I began to look for what it was, and I think I may have found a potential explanation.
But for now , lets look at what happened in Diablo 3.
- Diablo invaded the High Heavens and was stopped by the Nephalem.- what is not really known in this story is that the Scrolls of Fate of Itherael mentionned that when Diablo would walk the High Heavens, the angels and the Heavens will be terminated. But, somehow, the nephalem prevented the Truth from the scroll to pass and defied the very prediction the Scroll made.
Itherael never revealed his views on the ultimate fate of the Eternal Conflict, despite it being written within the scroll. He did not reveal that it was foretold that Heaven would lose when Diablo himself walked through the angels’ realm.[4] However, the fate of the nephalem was not recorded in the scroll,[4] as they were not part of the natural order of Creation.[5] And as such, the fate the scroll recorded was defied, and Diablo defeated.[4]
- As a result of this, in Diablo IV, we learn that Heaven closed it’s doors and went AFK, probably because they didn’t know what to do with the Scrolls of Fate becoming obsolete, their future was unknown and they became without purpose.
Purpose, we find this elsewhere, in Shadowlands.
The Purpose.
It is said on the description of Oribos, that the attendants are guided by “the Purpose,” the belief that the Shadowlands follows a pattern laid out by the Arbiter in her infinite wisdom.
I think this " Purpose " might be similar to the Scrolls of Fate things in Diablo .
Now, where do all of this connect ?
to what broke the machine of Death.
In diablo, the Nephalem defied fate and are absent of it, thus leading to unknown conjecture and future incertain.We learnt that Azerothian, in WoW, are similar. They can change destiny, they can change fate. The can create new future neither to Light nor the Void foresaw.
And they defeated the Legion.
I think the Legion was not supposed to be defeated, it was not foreseen, and was never on the table, even Elisande said it :
=> youtube.com/watch?v=pft35f9V8YU (2:09)
She peered in all different future and never saw Azeroth winning.She never saw the Legion being defeated. And during the fight with her we " changed " the future she foresaw and she lost her certitudes and visions and was defeated.
We have several mention of “fate” in Legion.
" Sometime the hand of fate must be forced " Illidan
" Their can be no chosen One, only we can save ourselves " Illidan
" It shatters the bonds of Fate, that seeks to crush us all " Aegwynn
" Our survival was never in fate’s hand. " Velen
So now, back with the Purpose. If the Arbiter is planning like a machine, and having a “purpose” according to the possible futures, and dispatch anima according to it, and the presence of the Legion in this purpose was never off the table, having suddenly this future changed on a cosmic scale because of Azerothian would break the machine of death and broke the " Purpose "
So the Arbiter would turn off or sleep to " recalculate " everything and reboot the Purpose.
In Diablo the Heaven closed due to the absence of Fate, allowing Hell to invade sanctuary again, and motivating Rathma to call Lilith from the void.
In wow the absence of the Arbiter allowed Sylvanas to help free the Jailer from the maw…I’ll speculate as far as saying that the Arbiter will maybe want to get rid of us when we will wake up and won’t have good intention, if we are an unknown variable in her plan, it’s not in her best interest and calculation to keep us alive in her equation.Proof is, we’re walking, alive, in the lands of the dead, we’ve already tainting the shadowlands and disrupting the balance…
To finish, there was this whispers in early BFA beta that said :
" The burning ones kept balance. Lost now… lost. "
We probably made a mess 
=> wowhead.com/news=283179/the-whispers-of-barrowknoll-cemetery-battle-for-azeroth-alpha