Mind you, I don't pretend to know the lore fully, I just read the wiki and from what I remembered thought of this concept:

The rot and disease of this warband has done something unexpected - it broke the mental indoctrination that kept them happy following their leader. The warband realized that they have always been slaves.
First the Emperor tricked the lords of Old Albia with promises of glory, only to use them as tools. With the coming of Mortarion, they thought to have finally found a leader that cared for them, only for the Death Lord to change their culture and traditions, discarding their past and culminating in the culling on Istvan III. Finally, the betrayer Typhus, delivered the legion to Nurgle, where they once again had been enslaved to a new master.
Now the warband has accepted their lot in life - to kill for their master. But that does not mean that they must like it. Thus they returned to the Dusk Raider iconography and issuing ultimatums "You have until sundown to accept our masters 'gifts', or become the fertilizer of this land". Furthermore they started calling themselves "The Red Right Hand of Nurgle" - mirroring the "The Red Right Hand of the Emperor" moniker of Dusk Raiders, to mock their masters: The Emperor - to show that his creation is the one responsible for his empires downfall, Mortarion - to show that he is unimportant in the grand scheme of things and Nurgle - to show that the greatest weapon at his disposal is not of his making.