Here's my interpretation:
At 01:15 into the clip, a service ground tug towing an aircraft (Hughes 5615 if I heard correctly) is cleared to cross rwy 4L on Delta taxiway, which is parallel to / next to 27C.
UAL1113 is told to go around. They do so but sounded surprised, probably because their landing was going fine and they didn't see any incursion.
As they're being vectored back for landing, the supervisor / other controller offers the explanation they didn't explicitly ask for but probably welcomed (and perhaps their airline would require them to explain, though I'm not an airline pilot so this is pure speculation): that an automated runway incursion system called for a go-around due to the aircraft under tow (the 5615 tug), which didn't seem like the human controllers or pilots perceived as a threat, but for some reason the automated system did.
If all of the above is true, to me it's a bit like the airplane's traffic collision avoidance system, where the pilots obey a Resolution Advisory, contrary to ATC instructions, and then explain to the controller why once things settle down. I know KORD is a vast airport with past incursion concerns so I'd bet the warning systems are followed carefully.
Nice find Brendan - I'll be interested to follow if others weigh in to validate or correct my theory.
Mike
