Right before the snap, the safety covering Graham retreats into a single high position. This puts the responsibility of covering Graham on the LB Myles Jack. The other safety rotates underneath to cover Anthony Miller, who is lined up inside of Robinson.
At the snap, Nagy sends Miller across the line of scrimmage. This motion confuses the defense and they make a critical mistake. Instead of following Miller across the field, the safety signals the LB Jack to take over the coverage of Miller.
This leaves Graham completely unguarded...