So I know of this and in most cases the entire tribe was migrating. The Romans refused the migration, even lost twice to one tribe before defeating them the third time. Their entire tribe completely abandoned their homeland due to overpopulation and pressure from other tribes. It happened again and again.

Ironically, the tribe just wanted to move into Cisalpine Gaul. They had completely wiped out the Roman army in the north and Rome assumed they were about to be destroyed, but the tribes weren't interested in that.

Hundreds of years later after the fall of the Western Roman empire the pattern resumed, with tribes migrating great distances, reaching as far as North Africa.