The first grammarian of English to condemn you was was Robert Lowth, in the first edition of his grammar (1762) (Tieken-Boon van Ostade 2002), and as I show in my article, usage dropped significantly after that date. This suggests immediate influence of the grammar, but the reason why Lowth included a stricture on you was in his grammar may well have been that usage was so frequent at the time that his attention as a normative grammarian was drawn to it. This is how usage items frequently attracts criticism. The split infinitive, first commented on in the early 1830s (Bailey 1996: 248), is a similar case, and so is the modern use of like (see e.g. the blog Bridging the Unbridgeable on this), though I haven’t come across any comments against it in modern usage guides – yet.
Interestingly, usage of you was was corrected in the second edition of Jane Austen’s Mansfield Park, originally published in 1814 but reissued in 1816. This change was recorded by Kathryn Sutherland in her comparison of the two editions of the novel (Penguin edition 1996:415-424). The speaker is Fanny Price, who said “I thought you was against me”.
Henry Churchyard, on his Linguistics Page in which he also deals with Jane Austen’s language, notes that he only found four instances of you was in the novels:
the phrase “you was”, for which the statistics are Narration: 0, Lucy Steele: 3, Nancy Steele: 1.
For his analysis he notes that he drew upon freely available online editions of Jane Austen’s novels, and the version found in Project Gutenberg indeed shows the correction of you was into you were. There should therefore have been at least one more instance. (Jane Austen herself didn’t use you was in her letters.)
Bailey, Richard W. (1996). Nineteenth-Century English. Ann Arbor: University
of Michigan Press.
Tieken-Boon van Ostade, Ingrid (2002). ‘You Was and Eighteenth-Century Normative Grammar’, in Katja Lenz and Ruth Moehlig (eds.), Of Dyuersite & Chaunge of Langage: Essays Presented to Manfred Goerlach on the Occasion of his 65th Birthday. Heidelberg: C. Winter Universitaetsverlag, 88–102.