You're dealing with the phrase lo que fuera que ocurriese.
The subjunctive is called for because of uncertainty; something that was about to happen (in the future) is the topic of discussion in the final clause, and there is no certainty about the future.
The translation of the phrase in question, and that which follows, is "whatever happened was about to happen several minutes later."
By the way, the title of the book is La chica de nieve. In Spanish, only the first word is capitalized (proper nouns are also capitalized). And the title should appear in italics to offset it from other words in a sentence.
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