Now..confusing topic name perhaps...but it deals with how they're given a basic "theme" as to what they fit.
Country, rock, etc.

Exactly what causes the selection to be made? How does one tell from hearing music what it is?!

hrm...i'm willing to bet "country" came from it being invented out in the country, "techno" i know came from the word "technology".

"Rock" came from "Rock 'n' Roll," which was first coined by a DJ from Cleveland, Ohio, back in the 50s.

"Emo" came from the word "Emotion." Since emo focuses heavily on human emotion, the name makes sense. Since it's a punk derivative, it has a very identifiable sound.

"Rap" came from the slang definition of "rap," which is basically a talk/conversation/discussion. This defines the general sound/style of the music: the vocalist talks and discusses, as opposed to singing.