Nomadic tribes, including the Huns, played a huge role in the downfall of the Western Roman Empire. There are at least two ways in which this happened.
By the 370s, the Huns had migrated as far as Eastern Europe. As they moved, they displaced other tribes, forcing those tribes to move into Roman territory. This was one way in which the nomadic tribes helped lead to Rome’s downfall. As these tribes moved into Roman territory, they often came into conflict with the Romans. These “barbarian invasions” were costly for the Romans. Even when the Romans were able to beat them back, it cost a great deal of money to do so. This meant that the Romans were constantly having to expend money and resources on fighting nomads. This helped to weaken the Roman state.
The second way in which the nomadic tribes weakened Rome was by diluting the population of Rome and making it less loyal to the idea of Rome. As the Huns moved westward, the Romans decided that they could not withstand all of the nomadic tribes on their own. Therefore, they did things like making alliances with the Visigoths in hopes that the Visigoths would defend them against the Huns. As the Romans did this, however, they were bringing non-Romans into their territory and depending on them for defense. This weakened the cohesion of Roman society, helping to lead to the downfall of the empire.
In these ways, the Huns and other nomadic tribes helped bring about the downfall of Rome.