So it's a myth that loads of them tuned up late and p***ed, without tickets?
Too many people in too small an area. Sure, the police f**ked up by opening the gates, but it was kicking off outside the ground and they were fearful of someone getting seriously injured. Nobody at the ground on that day could predicted what was going to happen inside the ground. The communications on the day were crap, and the poor instructions given to the police/stewards regarding opening the pitch gates to relieve the pressure definitely didn't help.
The biggest issue with Hillsborough was the cover up, but the catalyst for the whole event was the surge or people outside the ground just before kick off, but then that always gets overlooked.