tldr; The socket didn't properly fit the wrench and they had to manually hold the socket on the end of the wrench. One man dropped the socket, which fell through a small gap between the walkway and missile. The torque wrench was large enough that it would not have slipped through this gap if/when dropped.
It may seem like micromanagement, but small seemingly meaningless deviations from procedure can add up to big problems, particularly when dealing with space.
tldr; The socket didn't properly fit the wrench and they had to manually hold the socket on the end of the wrench. One man dropped the socket, which fell through a small gap between the walkway and missile. The torque wrench was large enough that it would not have slipped through this gap if/when dropped.