Over on one of the sygyzy boards there was a plausible explanation.

Frank Condon questioned the initial 4.8 (his lab is roughly in the vicinity) since he hadn't felt anything. In response, a poster replied that she had read that the military was conducting exercises in that area.

I bet the seismologist at USGS reviewed the initial data and came to the conclusion that it was man-made.

-- still it would be nice to get a little explanation on the USGS site to reduce the mystery. Now you see it - poof! - now you don't!