Was it ever explained why the empire ordered the men at Tyrant’s Hold to disband without pay? I get that the lore notes are there to tell us why it’s infested with Grobles, but it doesn’t make sense as to why they would just disband them without pay, unless the Justicar who sent the proclamation is possessed by an Aetherial.
Well you answered your question yourself. 
It is indeed done by the Aetherials, either from the capital or other city. It done simply to not have a fighting force against them while invading Cairn.
I don’t recall it being explained that the Justicar was an Aetherial though. That was just a logical guess.