In 1 of the Ned’s chapters it is revealed that he( & the whole court) knew that Tywin had called his banners & was raising an army at Casterly Rock post-Tyrion capture & Jaime’s flight. So why didn’t Ned send a raven/ messengers to Casterly Rock ordering him to stand down under the King’s Seal ? After all, at that point of time Ned was acting King( with Robert away hunting).