I'm confused about the use of imperfect tense (éramos) in this sentence below, rather than preterite (fuimos). I understand the knocking on the door as a specific, completed action that took place at a specific point in time, which I understand as characteristic of preterite (We knocked on the door/It was us who knocked on the door.). I understand the use of imperfect to discuss ongoing, continuous actions, so I'm confused in this example, because a knock is not an ongoing action that continues over time. I'm sure I'm missing something in my understanding; I'd appreciate some advice! Gracias!
Éramos nosotros los que llamamos a la puerta.