A: Humans mimic others speech (unconsciously) in order to fit in. EG: person from country A moves to country B. Both countries speak English. A few years later A moves home. People in country A now hear a country B accent when this person talks.
B: An isolated population's pronunciation will naturally drift away form that of the seed population. (this is how Latin morphed into French, Spanish, Italian, Portuguese and Romanian after the fall of the Roman empire)
In this example you have a population likely to come from several countries with different English pronunciation. While in isolation everyone unconsciously alters their accents to more of a local norm.