As far as I understand, lymph nodes not only swell if you’ve got an actual infection, but also if your body thinks it has an infection.
In your case, your dust-mite allergy (better put: your allergy for one or more of the excrements of the dust mite) is caused by your body over-reacting to these excrements. Your body fights those excrements as if they were bacteria or viruses. The body does this by activating all kind of immune cells to attack this “enemy”.
It’s the activation of your immune system that causes your allergic symptoms, as well as the swelling of you lymph nodes. As far as your body is concerned, there is no difference between an actual infection and an “infection” with dust-mite excrements.