I think some people don't realize that their ticket allows them to travel on any train and that they have to get the one they selected online or stick strictly to the itinerary they were given by someone else, so they rush to get the train and as soon as they see what platform they need and where it is then they lose all focus until they are actually on the train, after checking departure boards and asking 3 members of staff if it's the right train.
Another situation is the fact that people want to get to places as soon as they possibly can and will do their best to get on the next train even if that means it leaves in 30 seconds and they have to cross the station, they would rather run and risk it than wait for 20mins (ok, occasionally longer but normally no longer than an hour) and take it easy.
You also get people who have absolutely no idea what they are doing or where they are going and sit in the ticket hall until their train is announced, often distracted by listening to music, reading a book etc and then have to cross the station in less than 5 mins and getting lost on the way, getting held up on the stairs etc.
Some people just don't help themselves sometimes and just expect it to all fall together and work out fine, which it often does but also can easily go wrong very quickly. And the others who don't plan at all and just rush, rush, rush and end up getting caught out sometimes.
If everyone had some common sense or even some sense of direction then things will run a lot smoother, if you've never been to a station before, find your way and sit on the expected platform before plugging yourself into your music and keep alert and watch the departure boards/other passengers.
You've got a variety of different situations and they often all crash together and get in each others way, this is where delays happen and people miss their trains. The TOCs do all they can to try and control this chaos but people don't read posters (they may read them but not take the info in) or listen to announcements because some people believe it doesn't concern them and the people who do listen are either sceptical about whether to believe it or not or just get on with it.
I could go on all night but think I will stop now.