This is especially bad on Stansted Express services, both ex Stansted and Ex Liverpool Street (and to a lesser degree Norwich services) where the rear 5-6 can be showing totally full on the train loading information, whilst the front 3-4 carriages are very lightly loaded. I've been on trains which were like sardines at back with only a handful of passengers on the front.
People are just too lazy and don't want to walk down the train and would rather moan on social media that the train is overcrowded and like sardines and inhumane than bother to actually walk a little bit. It seems particularly bad in this country as whilst it happens on the continent, nowhere near as bad as here.
You can't do much about an end of line terminus other than is being done already (and in Stansted recently they tried an experiment with listing the train as serving platform 1B/1C (the furthest sectors) rather than platform 1 to stop people gathering at the back 1A part, and all it did was make a lot of people ask is the train at the front of 1A going to Stansted, a staff member told them no and to go to 1B/1C and passenger accused staff of lying and had a stand up argument with them!
Best ways against it are on multi-stoppers with a few busy stations, to try and direct passengers to platform entrances that lead to a different sector of the train than the rear of the train, or build stations in a way that encourage it, by feeding passengers to the front of platform rather than the rear that is crush loaded from lazy people at the terminus. But these things are not cheap or simple.