One of the most profound developments in European history was the Migration Period, which lasted from roughly 400 to 800 CE and which saw the Germanic tribes come out of southern Scandinavia and Germany and into what is now western Europe (the Roman provinces of Gaul, Britannia, Hispania, north Africa, and Italy).
There were doubtless several factors leading up to these migrations. Climate has been cited as a factor; in the winter of 535-536 written sources tell of an especially harsh winter and cloud-occluded sun (perhaps caused by a massive volcanic eruption). With crops failing due to colder temperatures, tribes would tend to move southwards into more fertile lands.
Asiatic and Slavic migrations would also push the Germanic tribes ahead of them, who would in turn push other tribes, like billiard balls knocking into each other. This is how the Franks ended up in Gaul (which then became France), the Lombards ended up in northern Italy (Lombardy), the Burgundians in eastern France (Burgundy), etc.
Now imagine a similar mechanism at work in a fantasy world.
In cold and snowy Trollheim, vast numbers of trolls and ogres dig their warren-like homes deep into the glaciers. As the climate grows warmer, however, the glaciers fracture and some calve into the sea as icebergs, taking the troll-warrens with them. Currents bring those icebergs into proximity of Scandia, “Mother of Nations”, which is inhabited by a number of human barbarian tribesmen.
As icebergs full of hungry ogres and trolls come ashore, at least some of the human barbarians decide that the time is right to pick up and move southwards, into the more civilized lands, where they begin their own raids and settlements along the frontier.
Not only does that give a reason for barbarian (or humanoid, for that matter) raids and migrations with an historical precedent, but also provides a ready-made excuse to have floating ice dungeons full of ogres and trolls. And that is a good thing in and of itself.