As explained in the book, it was simply “The Premise.” Like a lot of my favorite sci-fi, it tried to run through a speculative scenario as faithfully as it can to science, engineering and sociology. But was there any reason for it to happen in the first place? No. Stephenson explicitly says so and doesn’t bother cooking up some contrived reason. He wrote the book because the premise was entertaining to him.

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.

> Why did the moon explode without warning? Why?

If you take it as given that the moon is going to explode, it's guaranteed to happen without warning, for the simple reason that we do not know of anything that might cause that to happen, so even if warning signs were present, we wouldn't understand them.

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.