The video discusses what the Great Vowel shift is and some possible reasons for why the shift started at times. It mentions the contact of dialects in 14th century London(mass migration caused by the plague), conscious change to be more prestigious and fashionable, maybe some political identity formation to sound more or less French, and the printing which standardized the form of written language. The note on the vowel shift may have started from a conscious political movement that reminded of the audience-design and speaker-design style shifts which were discussed in the Investigating stylistic variation paper by Schilling-Estes. It is really an interesting hypothesis that much broader things such as politics may have affected more than one person leading to the shift in vowel pronunciation.