How exactly did the Manderlys receive their fief in the North? Did some Stark King hear of their fate and take pity? Or were the Manderlys actively looking for new land? A House migrating from one Kingdom to another never happened before or since then, right?

As I’ve discussed here, here, and here, the Manderlys went into exile with a good bit of portable income and were looking for not just land to settle on but royal protection (they were for all intents and purposes exiled fugitives), and the Starks were looking for a House with ready income to take over the Wolf’s Den and solve the tricky problem of the security of their eastern border:

“A thousand years before the Conquest, a promise was made, and oaths were sworn in the Wolf’s Den before the old gods and the new. When we were sore beset and friendless, hounded from our homes and in peril of our lives, the wolves took us in and nourished us and protected us against our enemies. The city is built upon the land they gave us. In return we swore that we should always be their men. Stark men!“

As for a House migrating, we’ve definitely seen some examples of this: the Blackwoods moved from the North to the Riverlands, and lots of Andal Houses would have moved from the Vale to the Riverlands to the Westerlands and the Reach.