Halflings are a peaceful people, who mind their own business and are more worried about what they will have for second breakfast than how to acquire wealth and political power.
Other races, especially humans, view halflings as unworthy of respect. Even the most venerable of halflings are considered little less than children. It’s no wonder that when a human kingdom seeks to expand its lands, halfling lands are the first to be taken, much like the lands where animals live are taken: no one would think they are stealing anything, animals don’t own their lands.
Halflings can sometimes be invited to live among humans, but most often are assigned reservations. Many halflings accept these conditions, since they know they are at a disadvantage, but others, more proud, perhaps a little too proud, decide to leave in search of other lands.
But there is less and less good land for a civilized people, so, many halflings have formed small villages in dark forests and underground tunnels. But the halflings are people of the prairie, of the open field, the lack of sun and space and freedom, has acted negatively in some of these communities.
Leaving behind civilized customs, such as tailor-made suits, elegant haircuts, and hot showers, but above all abandoning their pacifism, many have nurtured resentment against those who have taken over their lands; entire generations have given way to these new, aggressive and savage halflings, who form hordes to invade human towns, with the aim of killing and destroying, of sowing terror in human hearts, of bringing a bit of vengeful chaos, knowing that they won’t be able to recover their lands. Anyway, that is not what they want, they have become used to living in their unsanitary villages and their abysses without fresh air. Most likely, they don’t even have a goal: it is resentment, hatred, and anger which drives them.
Note: I am not talking about Tolkien hobbits, I talk only about D&D halflings and goblins.