I'll guarantee you, you have never been held in a queue for no reason. You might not know the reason, the staff holding you might not know the reason, but there will be a very good reason. Having been on the other side here are some examples,
Medical emergency at another station, someone ill on a train (happens a lot in big crowds)
Arseholes being arseholes at another station preventing train running
Police dealing with an incident (see arseholes above)
Train breakdown
Loss of power... and my favourite...
Fans on stairs at Ibrox Inner going mental "I could get on that fucking train, there's space at the door, ffs, your a prick, bet your a Tim, ya wank, etc, etc" all because he has to wait 4 minutes for next train. I use to explain the the train is going to Cessnock which is also rammed with bears and we leave a little space so a very limited number can get on board there as well, but by that time the rage has set in and they aren't listening to you. Similar to the Inner at Partick pre game, if you have a drink in the pub and want to travel at 2.45 to get to game on time guess what, so did a few hundred others and even more in town who left at 2.30, the trains busy and you will have to wait, its not the staff, the train or the systems fault you couldn't put the pint down and head over earlier.