I found an answer here [1], via a Reddit thread [2] (where the answers are apparently gone):
I won't go into details, but if the conductor wants to exit
the train and leave the doors open at the terminal, he needs
to walk to the next car at the last stop in order to open
the doors, which is why there's a short delay before they
open. If the doors open immediately at the last stop, then
it means the train is either going to go to the yard, or the
crew is going to manually key open one door per car (which
is only done if the train is going to sit there for at least
10 minutes, and helps the air comfort system).
Very interesting. I definitely always hear the conductor moving between cars at the last stop but I never connected that to being why the doors can't be opened.
I think it's funny that the train's computer knows it's the last stop and announces that fact to the passengers... but the conductor has to manually override some sort of system before he or she can exit the train.
[2] https://www.reddit.com/r/IAmA/comments/kx5fa/iama_new_york_c...