This might be something to ask the folks over at /r/linguistics as well.

I can't imagine there is a unified answer here. There are so many factors which can impact these changes, but for a broad look at changing pronunciations in the English language, this response from /u/bloodswan I would expect to be of great interest for you, especially as it does touch on regional variations later on, not to mention discussion of the historiography of studying pronunciation. Hope that helps!

I have a slightly different question- which accents have changed the most and which the least since audio started being recorded with phonographs in 1877?