Memes

There is an excellent book called "Alaric the Goth" by Douglas Boin that goes into a great amount of detail into the cultural shift. A big shift was when Honorius stripped freedom of religion away, this served to move Roman society towards Christianity and reinforced the view of the tribes as barbarians.

And correct, actually incorporating non-Romans into the military was a great way for those tribes to attain a stable life after since they enjoyed the same benefits as Roman soldiers for their service. When that was stripped away, they also would be subjected to terrible treatment and often had poor provisions.