(Down island (East) was much more densely populated and favoured by summer visitors, whilst Up island (West) had many more original inhabitants and was much more rural
A big factor to consider when discussing the cause of these differences in pronunciations in Martha's Vineyard is largely down to the attitude of its inhabitants;
The heaviest users of this type of centralised pronunciation of diphthongs were young men who sought to identify themselves as native Vineyarders, rejecting the values and speech style of the mainland.
The fishermen in particular also resented the influx of wealthy summer visitors and were antipathetic to their presence as they believed it infringed on their traditional way of island life. This in turn encouraged the Vineyarders to establish a somewhat non-standard dialect and retain their social identity.
The tight knit community subconsciously ensured that they created a linguistic divide between them and us. The fishermen were seen to epitomise desirable values, which in turn caused other Vineyarders to adhere to a similar style of pronunciation.
For these Vineyarders, the new pronunciation was an innovation. As more and more people came to speak in the same way, the innovation gradually became the norm for those living on the island and was established as a dialect.
Therefore, there seems to be enough evidence to state that generations, occupations or social groups might be a big factor in language use as a sociolinguistic consideration.
A suitable hypothesis for further investigation is "People with a more positive attitude towards Martha's Vineyard would show more centralization than people who had a negative attitude towards it