There are currently three main theories about how the language of Ashkenazi Jewry developed; the truth is likely to include elements of all three.
Read more on Forward: http://forward.com/articles/202706/the-origins-of-yiddish-part-fier/
The Origins of Yiddish Revisited https://mosaicmagazine.com/picks/uncategorized/2014/07/the-origins-of-yiddish-revisited/
About the author: Philologos, the renowned Jewish-language columnist, appears twice a month in Mosaic. Questions for him may be sent to his email address by clicking here.