I am not the author, but I have the answer.

>> When you leave the station, people in the door look around.

> Dear friends,
>Please explain to the foreigner, what is the joke at all?
> thank you in advance.

They turn around to see if anyone is following them. And if there is someone in the back, they hold the door so that it does not get into the cage of the backward. And so that he who is behind does not spend an extra time (even if a little) on re-opening the door.