Probably just a good old fashioned presence sensor. If the sensor is triggered, there's a "box" there, and the robot does a pre-programmed set of actions. The robot would place the box on the conveyor nicely, but if the man's head and chest stuck out differently than the box does, robot doesn't care. It goes to the programmed position regardless. By the time it encounters enough resistance to trigger the collision detection, the damage has already been done.