The action of knocking on the door is seen as a sudden and completed action, but the verb in the imperfect "'éramos" is saying "who were" the ones who knocked, and that action is seen as "descriptive", this is why the imperfect was used. The verb for "knocked" is used in the Indefinido:
Nosotros éramos los que llamamos a la puerta.
It was us who knocked on the door.
I hope this clarifies it.