Confused by the imperfect subjunctive again.

[createdamadman]

Okay, so I've been asking a lot of questions on here regarding different uses of the imperfect subjunctive in a book I'm reading, and so far all have been answered with great responses, however, more instances keep arising which don't seem to be explained by anything I've learned yet, and which I can't seem to find answers for online.

This time, I have the passage below, and don't understand why imperfect subjunctive is used in the first sentence. I'm sure from the context that the events have definitely happened. It is basically reading as "Since this has happened, this has been the case." I'm again at a loss as to why the preterite isn't used here.

"Desde que esta pregunta se formulara en su interior y cobrara vida tras las palabras de Pascal, el tiempo había tomado una nueva dimensión para él. Si antes se concentraba únicamente en disfrutar cada segundo que pasaba junto a Nina y esperar con ansias que llegara la noche para volver a estar con ella, esa mañana, mientras caminaba hacia ninguna parte, sentía una angustia vital insoportable."

[Rusty]

There are a few reasons given for this usage here. From ancient language being revisited to regional usage and relative clauses introducing new material, it has its place.
Look here, too.