The return of the test today caused a bit of a stir. It came down to what really causes great sound changes like the Great Vowel Shift. I told the truth as I knew it - that nobody really knows why all English's vowels shifted sometime between 1400 - 1700. There are dozens of theories.
Several things are notable about the era. People moved to southeast England from all over the country, to escape the black plague. There was a lot of social mobility for the same reason. The aristocracy who had spoken French for years started speaking English again. But people who wanted to move up spoke like them. During one of the many wars between France and England, it became a bad idea to sound too French, and people tried to sound more British. etc. etc.
The question comes up because of the Northern Cities Vowel Shift, and whether that is an entire language shift, or just the creation of a dialect. In the case of the NCVS, which is happening now, almost nobody is even aware of it; it is surely not happening for social reasons. I can see people changing their vowels to be similar to those around them (I am doing it as we speak), but, there is no social prestige involved in this particular shift, that I know of. I presented it to my class. Why do people do it? I don't know.
This is the story I present to them. We as linguists try to explain human behavior, and humans are indeed doing things that surprise us, defy explanation. The introduction of a new language would seem to tax people, give them too many rules to remember, stretch their possible number of vowels, etc. But why would this happen in the North before it happened in Texas? Or, why would Canadians refuse to budge or change anything? Because these are geographical questions, my interest is piqued. But I don't really have answers. More about this later, I hope.