Combine them and the Liberals and you get a population that at least believes itself to be largely left of center so actually piss off.
That is a very slim margin, and a whole lot of voters are fence-sitters who might vote left or Conservative. If the Liberals were the NDP... well, Trudeau was not set to win. Because people hated him, and wanted something to his right "for a change".
You mention the pattern. The pattern is a pendulum swinging back and forth. If you think it's been waiting to go hard to (one's specific version of) the left all along, that's unscientific. The only actual control activists can possibly have is the pace and distance with which it swings.