He actually does suggest a reason in the book, which is a primordial black hole passing through it.

Yeah I get that. And I also understand that, given their predicament, the characters in the story have little time or opportunity to consider the matter for us.

What annoyed me was that the "premise" is (a) extraordinarily unlikely and (b) its causation doesn't form any part of the plot. Building a hard SF work on such a foundation is a big ask of the reader - although I'm sure it was entertaining for the writer. Thats my problem with the book.

We do actually have a theory of someething that might cause it to happen; primordial black holes zipping through space, randomly going through the moon (and continuing their way as if nothing happened, because that's how physics would work out). However, it's only a theory and depends on in-flux research on early universe cosmology — and the black hole could equally randomly zip through Earth rather than the moon. Or pass in the space between. Or pass between Earth and the sun. Or…

The book AFAIR does point out it "might have been a primordial black hole but they don't know", which matches what the situation would be IRL.