The design of the Leppings Lane end was likely to encourage such a crush.The area was divided into three pens, with access to the central pen via the tunnel and side pens by the ends. Since access to all three was from the area behind the turnstiles with no further control, it would be impossible to control numbers in each pen even before the gates bypassing the turnstiles were opened. Because the obvious way in was through the tunnel and most people wish to stand behind the goal, most would attempt to enter the centre pen and few would enter the side pens to which entrance was less obvious.
At some other grounds this would be unlikely to happen. At Wembley when there was terracing each standing area was separately ticketed. At Anfield on the Kop there were no fences dividing the standing area into pens.
Any individual in a crowd has little control over his movement. That has led to the other crush disasters such as Ibrox and the wartime accident in the London Tube. To suggest that those who formed the crowd were responsible is absurd. I am sure that given information they would have acted responsibly, but they could not possibly have known of problems at the front; indeed they could suppose that those controlling the ground had a duty to ensure that the pens were not overfilled.
Regrettably, even in these days of all-seater grounds, it could still happen, as all clubs do not issue tickets for specific seats. Fortunately we no longer have fences, so the results are unlikely to be as serious.
I trust Mr Roberts will accept that a crowd will always act in this way (rather like water) and ceases to be a group of individuals with individual will.