The Autokrats were satisfied with their suspicions confirmed. After finishing off the grain hauler with another volley (Surprisingly, Regnal survived this, and would meet his own gruesome fate far later- but that's another story) the Autokrats met once more, and elected a High Autokrat among their number- as far as they were concerned, they were in a state of war.
We have no record of the identity of the High Autokrat. Their position, history, statements, gender, and fate are unknown. The Death Korps of Krieg were thorough in erasing this hated figure from memory. All we know was that there was a High Autokrat, and that this figure would openly declare secession from the Imperium of Man.
At this time, seven Astra Militarum regiments of Kriegers had been raised, and were posted on the world. They had been staying garrisoned in case of tyranid attack, and awaiting orders that would have presumably accompanied the Administratum delegation.
As part of their duties, they were to gather supplies. This included food for campaign, begrudgingly supplied by the planet.
The hivers of Krieg had never been strangers to hunger. It is the same on all hives- due to corruption, inadequate transport capacity, or the simple structure of billions contained in such a small space, malnutrition is rampant. There are always too many mouths, and never enough meals.
The hivers of Krieg had known the shock of having the Imperium fail them, and now faced famine. The Death Korps of Krieg stated simply that the population rose in rebellion against the Imperium, and by extension those Imperial Regiments as well, but I feel that this misses a step. To editorialize, I imagine that the High Autokrat (Being a cunning sort) let this stew a few days. Let the rations dwindle, let the people wonder what's going on- perhaps even attempted to rather publicly deny the Imperial Regiments their supplies, and suffered equally public rebuke.