I've heard some people posit that the reason a large amount of idioms in American English come from sports terminology (e.g. "ballpark figure" or "the whole nine yards") is due to the "competitive nature" of Americans. While I'm not completely bought into the idea, is there a currently accepted theory for classifying the determining factor of idioms in a culture?
The most common way for new languages to form is fairly regular. When two cultures meet, the dominant culture's language forms the basis for a pidgen in which those in the less dominant culture try to accomodate the dominant language while using grammatical structures, vocabulary, and metaphors/symbols from their own language. Once a complete language develops from these, it's often called a creole which has elements from both languages, including the metaphoric/symbolic imagery from each--(at the same time, the creole simplifies the grammar/syntax of the original). So the idiom is usually a metaphor which meant something concrete in the original language but which has taken on a more abstract meaning in the creole--which is then a real language. English went through this process, absorbing symbols and metaphors while simplifying forms. Sometimes individuals create idioms which go on to be applied to many things; for instance, Shakespeare: wild goose chase, the crack of doom, as pure as the driven snow, and many more. America added many: well healed, on the wagon, off the record, +++. It's a fascinating study.