From The Cambridge History of the English Language: English in North America, p. 332,
by John Algeo, via Google Books:
When thou, thee, and thine
dropped out of the language in the
early Colonial period on both sides of
the Atlantic, speakers felt a need
nonetheless to distinguish between
singular and plural forms. The
earliest attempt was simply to make
verb agreement do the work: speakers
would say you was for the singular
and you were for the plural.
Beginning in the eighteenth century,
this sensible solution was met with
heavy resistance from purists, and
you was became heavily stigmatized by the end of the nineteenth century in
America (though it has by no means
dropped out of colloquial speech in
the United States).