Tis a good one. My reaction is self-centered, of course. That the Munk Debate shifted the discourse, as Trudeau pretty much put a stake into the "Justin is not ready" argument. But a closer look at the timing shows that that debate (not so much the foreign policy stances, but the readiness) may have impacted the rise of the Liberals (late Sept/early Oct) but not the collapse of the NDP.I would really like to know what happened in mid-September in Canada pic.twitter.com/eq1bLC5NUX— Stathis Kalyvas (@SKalyvas) October 20, 2015
What did that? Many folks who indicated support for the NDP in August became either Liberal or Bloc voters in September. The niqab issue definitely hurt the NDP more than it hurt the Liberals, with those that care about that in Quebec moving to the Bloc. But that probably does not explain why the NDP got annihilated in the Atlantic provinces. TPP cannot explain it since that agreement did not bubble into an issue until October.
It may be that the public got more attentive once people returned from summer vacation and that the NDP alienated its base by shifting to the centre. A key late August/early Sept move was the Liberal stance to run budget deficits. The NDP pushed hard against this, but that might not have played well in those parts of Canada that might want more government spending--Quebec and the Atlantic provinces. The Liberals ran to the left on key issues this fall, and it seems to have worked, capturing much of the "progressive vote" despite supporting the Patriot Act-ish Bill C-51. Perhaps the NDP moving to the center and then back to the left alienated heaps of supporters?
I expect real Canadianists to figure this out via survey work. I can only guess.