You are asking me? One thing I'll say is that this did not remind me of last year's Alberta polling cock-up. The B.C. NDP are not an untried bunch, and the charges of radicalism that were flung at them are par for the course in a B.C. election, so I can't see them having had much effect. If I were to guess, I would say the turning point came when Christy Clark came out against the Trans-Mountain pipeline, essentially mirroring the NDP "no-pipeline" stand and taking away their most important policy contrast. But I'm not even going to try to guess.
PS. Congrats to Andrew Weaver, the first Green Party MLA ever elected to the B.C. Legislature.