Earthquakes can set off other earthquakes. Earthquakes are a result of built up tension between faults, so the magnitude of one earthquake can hit another fault and release the tension - and walla, you have another earthquake.
Yes, that is how the spokesperson from the USGS explained it. In a press release. Natural or not, I thought it was odd.
Yes, that is the book (Behold a Pale Horse) and author. I thought it was interesting and seemed more plausible given the author's military background. I read it before 911, Katrina, and FEMA but have considered a reread in light of those events. It's a unique perspective, if anything.