I would say that it's a nebulous and organic process that settles into particular pronunciations. One thing a lot of people don't realize is that the British accent is actually considerably more divergent from the one that was common to America and England in 1600 (I can't seem to find the source right now, but will edit if/when I do). I'm not saying that it's right, just that it happens and that there are valid reasons for it happening.