Dwarves are recognized as a strong, resistant, resilient and stubborn race.
When you get those "qualities", they are a very strong threat to any evil (and even some civilized races).
So, when some evil appear, he tends to go for the strongest enemy and convert him to his side or finish their existence.
Or in the case of a "civilized" race, they usually tend to clear the "noise/different" that mess with the way of think/act. (The same as the Europeans did in the Americas). And, let's say that the Dwarves have a unique way of living that doesn't take too much outside influence and "don't care" too much about the outside culture too...
About the Tolkien reference:
Unlike Elves and Men, the Dwarves are not counted among the Children
of Ilúvatar. Their creator was Mahal, known as Aulë the Smith. Aulë
created the Seven Fathers of the Dwarves somewhere in Middle-earth,
from whom all other Dwarves are descended. Aulë though, did not have
the divine power to grant independent life to any creation; Aulë
repented and confessed to Ilúvatar and promised to keep them; the
voice of Ilúvatar though spoke to Aulë and agreed to grant them true
life, and include them in His plan for Arda. Ilúvatar granted the
Dwarves life, and therefore they are known as the Adopted Children of
Ilúvatar and they were to awake after the Awakening of the Elves.
as you can see, they are really "stone heads" that even "challenged" god (without knowing but ...).
As developers/gamers/DM/Bards... usually use some base for reference for their "tales", you can see that almost (every) time that there is a Dwarf, he is the bad-ass of his group (usually their leader or at least has his word take in consideration).