This is an issue with entrances right at one end (as typically at termini), the increasing trend to open trains just a few minutes before departure, and no information about how long to departure until the door chimes go, and then it's instantly 'stand back and slam doors'. The days of whistles being blown 30 seconds before departure are gone. Once you've actually missed a departure because of this as you are halfway walking up a train to more space at the front (DLR at Tower Gateway, looking at you), it just encourages passengers to leap in the first door they come to.
Of course, once people have packed in to standee positions at the rear, it becomes difficult to move forward anyway. The industry still hasn't understood that the statements about how easy it is to move along the train with wide gangways involves the same floorspace that another part of the industry has proudly announced as part of the train's great capacity, with passengers jammed in at 6 to the square metre.