The NDP surge from the summer has sort of fizzled out I think. Public perception of Tom Mulclair as "The not Harper guy" has been dropping and Trudeau has also started looking more and more Prime Minister-like, more confident, more able to run a country, not just a boy with a famous name, etc. and Mulclair has been Mulclair. You know what you get with Mulclair, there's nothing new there. I think that there are a lot of left-leaning voters out there who are riding public perception and so are more prone to switch ship from NDP to Liberal or vice versa, depending on the circumstances, strategic voting considerations, etc.
So I think the Liberals just got some good momentum due to all those factors and now they're looking pretty damn good. Trudeau's also been targeting conservative voters in these last couple days and that seems to be working too.
Do I dare say that a Liberal majority is possible, if this keeps up? Not likely, but it actually seems possible now, if conservative and NDP support keeps dropping.