Does anyone know: Where do accents come from?

Where do accents come from? I'm thinking of large-scale accents, not personal idiosyncrasies; and I'm wondering about their original development, not the transmission of established accents to new speakers.

Take just one example: the Long Island pronunciation of "or" as "ar" of which my girlfriend never lets me forget. "Orange"->"Arange"; "Horror"->"Harrar" (or "Harra"); "Florida"->"Flarida."

Where does that accent come from? Is it a confluence of different speaking styles/languages? Is it a remnant of some original language that has survived? Is it the weather?