In a 2-2 game with 5:32 remaining in the third period between the Montreal Canadiens and the Detroit Red Wings, Luff tried to dump and chase but a Montreal defender batted the puck towards the board. As Luff went to retrieve the loose puck, Slafkovsky drove Luff into the wall from behind pretty dangerously, leading to a scrum.
NHL's Player Safety deemed that the rookie had enough time to assess the situation, saw his numbers, and did not need to drive him into the boards the way he did. Their decision making process stemmed to three points: 1) it was boarding, 2) Luff suffered an injury on the player, 3) no history of suspensions and fines for Slafkovsky before the incident