US intervention in the Patriots' Rebellion on the Canadian side seems like the best PoD, but it's still hard. (And it would require US intervention; there aren't enough Canadians at the time to win independence without help even if the whole place was united in support of the rebellion, which it wasn't.)
The two problems I see are that much of English Canada was settled by loyalist refugees from the US, who were, well... loyal, and that once responsible government passes, Canadians are ruled with a very light hand from Britain, and that doesn't lend itself to alienating the Canadian population.