Actually, that is quite historical, for the Magyar relocation to Panonia happened as a result of the defeat of their ruler, Levedi, grandfather of Arpad, by the Pechenegs (who, in turn, were fleeing westward after suffering defeat against the Khazars). It was after this defeat that the Hungarians moved westward, first on the Nistru (Dnester) at Atelcut (Etelkuzu), with the Pechenegs on their heels, and then accross the Carpathians and into Panonia.
Thus, I would like to suggest that, after a tribe is conquered (whether by another tribe or by a feudal state, does not matter), if possible, there would be a chance for event troops spawning, which would allow that tribe to relocate to a new area.