I contend the 2015 campaign was the dirtiest in 50 years of following campaigns and the Liberals were the dirt, dishonesty and deception champions of the campaign.  Liberals brazenly lying about the Harper government record is not the same as Tories telling the truth about Trudeau's verifiably anorexic resume and bizarre utterances.  .

In our 3 party federal system when over 60% of electors want a change of government they get it.  The governing party takes 30-40% while the other 2 split 60-70% and the bandwagon effect gives one of the opposition parties the highest vote %.   

The other major factor is whether leaders are seen to have exceeded or failed to meet expectations during the campaign.  Trudeau exceeded them in 2015 which, with a desire for change over 60%, led him to victory.

Poll numbers mean squat 2 + years out and less than squat vs leaderless parties.  This changes today.  P.S. The averaging of recent polls has the 2 main parties 5% not 20% apart, not that it matters.