In your first image, #9 is ahead of the play, and covered by Wotherspoon. Koepke is on #74, level with the puck. However, by the time the puck carrier crosses the blue line (second image), 9 and 74 have swapped spots. 74 is F2, and 9 is now trailing the play, and is F3.
As the backchecker, Koepke has switched men, and is chasing F3 (not the puck). In most defensive systems, I'd say this is the correct play... provided Wotherspoon sees the same thing and also switches coverage.
Since I don't know the B's system, I'll hold off on assigning blame to the players, but will absolutely give the finger to the coaching staff. By this point in the season, understanding that coverage should be second nature to the players by benefit of many reps in practice.