Of course, I am sure that the queueing (sp?) strategy is rather complicated, but at some point, a general principle might be: "allow one plane to be very late rather than delay others unnecessarily".

Because after a certain amount of delay, it makes no more difference to the passengers (in theory) to have another 15 minutes -- versus causing a whole load of other planes (new passengers) to also be delayed.

but this is just generalities -- like the FAA program to spread the pain of weather delays, it lies in the scheduling computer details I'm sure.