Your logic is not without reason however it is not necessarily the sole reason for a surge like this.
The momentum and movement of a crowd that size into a confined space was a job for the match commander and those in charge to avoid in which they failed.
Can you imagine leaving with a large crowd at the old hampden and only one of the exits was opened when the 20,000 or so in your section coming at your back off the terracing were expecting all the exits open?
There would have been a surge/push for sure.
The point is that most of the liverpool fans that day did not know what they were walking into and were reliant on decent instruction and crowd control from the police and stewards yet were herded into the fatefull pens.