Apart from the Zombie God Cult's reason, the higher-ups in Cloud Gorge Base really couldn't think of any other reason.
After all, zombies wouldn't just leave without cause. Living creatures, for a zombie, are like air. A person gasping for breath yearns for the very air they can easily breathe in when it is present. They wouldn't just leave and forsake the air that had just reached their noses.
Watching the vast scene of zombies leaving, the soldiers on the second line of defense were left speechless, and behind them rose the deafening chants of the Zombie God Cult members.