I have an answer for this! From the Maelstrom company leve in Mor Dhona, "Go Home to Mama":
The Yellow Adder leve "Get Off Our Lake" and the main scenario quest "The Rising Chorus" also refer to this. It seems like a lot of the Imperial forces have been left behind enemy lines and are trying to survive until the airships of the motherland come for them. They'll be here soon, right? Garlemald wouldn't abandon its loyal soldiers, right? ... Right?The supply route blockade is having an effect; the Garlean forces in Castrum Centri have sent a skulk of men to the shores of Silvertear. They forage for airship debris like children on a morel hunt, unaware that they have wandered too far off the apron strings of the Empire. In the name of the Maelstrom, secure the wreck and teach the sons of Garlemald a harsh lesson.