Let’s remember that during the 2015 federal campaign, the NDP were the frontrunners. Right up until about a month before the election. It was about that time that the Liberals started getting worried. Liberals don’t think the NDP are a viable party to govern in Parliament, so they assumed that NDP winning would actually result in a Conservative win. They use this belief to scare left-leaning voters to get behind them, saying that if people vote NDP, they will split the vote.
But it takes two to tango.