1/60 minutes doesn't make much of a difference to >16 hour flight? If the decision was made much earlier they could have told passengers at checkin.
It's not about the length of the flight though. It's about the length of time you have all the pertinent information about the flight (final passenger and cargo loadings) and most up to date meteorological conditions. As it will incur cost if you decide to deny passengers boarding, isn't it best to leave that to the last feasible point in time. I am sure at DFW the station manager and pilots are well versed in dealing with this particular issue and knows exactly what they are doing, particularly with that slightly later AA flight to LAX that connects with SYD and BNE flights.