Yeah it was freaking obvious....
It was less fan theory and more like common sense.
Blizz said the wheel of death broke during legion. We know that ursoc was sent to ardenweald after we defeated him in nightmare.
So now we know it wasn't at the start of legion.
So now what significant cosmic events happen during the rest of legion.
We defeat Gildane and revive illidan, okay cool story.
We defeat kiljaede, again nothing worthy of breaking the arbitor.
Then we fight a freaking DEATH TITAN. Not only that, but we learn that the nathrezime we're actually loyal to death and not the legion.
This means that argus was most like twisted for eons by the highest ranking members of the legion and then aimed at the arbitor upon his death by our hands.
Also keep in mind sylvanas was also in league with the jailor and she was all in for the legions defeat.
And ultimately the energy that hit the arbitor was very similar to the red energy that argus exhibited.
So again, this was extremely obvious to anyone who payed attention.