On the Fact that All Throughout the Recurrent Wars Between the Scots and the English (Post Roman Rule), Marauding Bands of Outlaws Were Terrorizing Both Sides and Carting Off Amongst its Immense Booty Poor Farm People Later to Be Used as Slaves

Ironically most of the slaves came from northern English settlements (England being more noted centuries later for their enslavement of the Scots and the Irish). This was likely do to the greater amount of farming in England than even in the Scottish lowlands. So an opportunity thing, in other words.