It's a safety procedure: if equipment is faulty, you lock the controls with a special device to render it unusable until it is serviced, and a tag accompanies the lock to show when the service call was placed. If locking is impossible, just the tag will suffice.

LOTO or lock out, tag out is a safety practice of physical locking out any and all energy sources attached to a piece of equipment. Gravity, electrical, chemical, potential, pneumatic, hydraulic. You put a lock on it with a tag stating your name, date and typically a reason. You keep that key for that lock so no one else can energize.

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.