Re-watching highlights from the game:
That 3rd Leafs goal, Marner starts from down low on the left hand side in the Leafs zone. Tyson Barrie starts on the right side, but turns to the inside and starts skating back towards his own zone as soon as Marner starts to skate with speed.
As soon as Barrie turns around, he sees Mark Barberio already pivoted and skating backwards, and already on the right side of the ice. I don't know if something was said or gestured, but it sure as hell looks like Barrie thinks Barberio is going to play that right side on Marner while Barrie himself shifts to the left, and Barrie looks confused AF when Barberio doesn't even attempt to play Marner and instead sticks to his man who is literally just a decoy outside the blue line.
Both defensemen are to blame for the goal, but I think it's common sense for Barrie to assume that given Barberio is already there, he should be playing Marner while Barrie acts as the safety valve and they rely on their back checkers to watch the incoming forwards.
Barberio just had himself a heinous game.