Why do the Huns seem to disappear into the annals of history following their sweep into the late western Roman empire? Why did they stay or come back at a later date to take a chunk of the dying empire for themselves like so many other groups?

What you have to realize about steppe empires that seem to appear suddenly, invade and conquer vast spaces, then disappear just as suddenly is that the name of the groups is most commonly just the ethnicity of the leadership, and that the vast majority of their forces consisted of other tribes that hopped on after the headliners started building something interesting. Turks, Jurchens, and Manchus would swear allegiance to Genghis a few centuries later and all be labeled Mongols when they fought under the Khans, then return to their prior ethnic identities after the fall of the Mongol empire. This is a regular process on the steppes, which historians and anthropologists have labeled ethnogenesis. (One can arguably see a similar process, more slowly and less direct, with almost all ethnic identities). See Peter Golden, The Peoples of the South Russian Steppes.

We have very little information about the Huns, beyond a generic steppe archetype of a group of horse archers marauding the civilized world, and almost zero information about their existence back on the steppes before they invaded the Roman Empire. So it is hard to pick out what their origin was, or what constituent groups became Huns before or after their invasion. The Hun empire lasts just 100 years from their first major moves, to the death of Attila and disappearance from history as an important force. In all likelihood, the actual number of "true" Huns was relatively small, on the order of a few thousand, with the majority of Attila's forces being made up of other steppe tribes that were conquered or allied with Attila, and people's he had conquered up until then. There is little historical consensus, but what likely occurred is that the constituent ethnic groups that went by the label "Huns" when they were lead by Attila and co. reverted to prior ethnic identities once the Hun empire fell and the name Hun lost its prestige value. Even the remaining Huns probably latched onto other ethnic groups, or simply melted into the general populace without much impact.