Which is where the advantage of a nomadic culture comes in. If the local sedentary people get huffy, the nomads can up and leave. They don't have to just sit there and take it. The nomadic elves in this particular universe have the advantage of plenty of forests and wildlands to retreat into where humans can't follow them.
While this isn't necessarily a criticism of the
Dragon Age setting - which has shown itself to, in many ways, ignore the way history works in the real world - I would point out that in reality, nomadic societies are not independent of local food resources. Whether they get by by grazing their herds, hunting local fauna, or some combination of the above, they still rely on local food sources. In both cases, those are most easily obtained through the acquiescence of sedentary authorities, should they exist; for example, hunting and grazing on lands specifically set aside for those purposes, as many transhumant groups in modern Africa do.
Since the areas capable of supporting a nomadic society are necessarily limited, most migrations take place within a relatively circumscribed territory with minor seasonal variations. And since the boundary between survival and starvation is so thin for many nomadic groups, any interference with local food supplies can be catastrophic. Burning off a grazing area or slaughtering local game are both time-tested means of really screwing nomadic tribes.
So say this tribe gets into a squabble with secular authorities and picks up sticks for avowedly greener pastures. Those secular authorities would have every motivation - and, generally, every ability - to simply denude those other pastures as well. (You mention areas where "humans can't follow"; while we've seen that some forests in the setting are both areas plentiful with game and difficult for humans to access, this is very rarely borne out in reality.) It's just me, but I'm not sure that that leaves the migrants any better off. In fact, it's probably a lot worse: