Actually my saying it was a trip and not a slew foot is because of my ability to understand the rule.
52.1 Slew-footing is the act of a player using his leg or foot to knock or kick an opponent’s feet from under him, or pushes an opponent’s upper body backward with an arm or elbow, and at the same time with a forward motion of his leg, knocks or kicks the opponent’s feet from under him, causing him to fall violently to the ice.
While keeping both his skates on the ice, Tavares hit one of Marchand's feet, causing him to fall. That's a trip.
The league is trying to prevent slew-footing because it involves both your feet getting swept out from under you, which often results in slamming down on your back and hitting your head on the ice. None of those things happened with Marchand.