It's actually a lot more like The Martian than you might think. Most of the book is about how hard it is to live in space.

It definitely tickled some of the same parts of my curiosity. Grinding hard sci fi.

Something hits the moon with enough force to cause it to break apart. He doesn't speculate at all what it might have been. I think he'd say it doesn't really matter - It happened, now what?

In the book the initial assumption was that it was hit by a tiny primordial fast-moving black hole. They never really follow-up on it, being distracted by issues of survival.

Sorta-kinda-not-really. The closest thing to an explanation in the public excerpt [1] goes like this:

The most generally accepted theory was that the puff of dust observed by the Utah astronomer was caused by an impact. That the Agent, in other words, came from outside the moon, pierced its surface, burrowed deep into its center, and then released its energy. Or that it simply kept on going out the other side, depositing enough energy en route to break up the moon. Another hypothesis stated that the Agent was a device buried in the moon by aliens during primordial times, set to detonate when certain conditions were met.

[1] http://www.nealstephenson.com/news/2015/04/13/seveneves-exce...

Not really, and the scenario has some other problems too[1]. But all sorts of books have fantastic premises, just look at Lord of the Rings! I can accept an awful lot of improbability in a book's setup as long as the plot given that premise makes enough sense. In Seveneves it did so I was happy.

[1]http://hopefullyintersting.blogspot.com/2015/06/seveneves-an...

I spent a long time pondering the premise of the subsequent planetary bombardment. Would the moon fragments really bang into one another the way they do in the book? Would it lead to massive fragmentation and ultimately the aforementioned bombardment?