Blaming 'drunken' fans and those with no tickets is a bit of a non-starter as, at the time, those planning for games knew there would be those elements in the crowd and they had a duty of care to plan for that and protect all fans. At the time, that type of fan behaviour was normal and had almost caused an incident the year before.
It's like planning for a modern festival, you know there will be a proportion of festival goers who will drink too much and/or take drugs. You plan for that both for their safety and the safety of others at the festival. In both the Green and Purple guides for event planning in the UK there are significant passages about assessing the nature of the crowd that will turn up to an event and how you should alter your planning and resourcing to reflect this.
The other thing to remember is that individuals in a crowd have limited information and can't always react in the most appropriate manner. People as the back pushing to get in won't know whether there's a solid physical barrier ahead or it's just people dawdling, holding the crowd up and by the time they do know, it may be too late for those at the front.
Declaring an interest, I had relatives in the crowd at the Leppings Lane end that day and have worked with members of the emergency and health services who were on duty at the ground and the receiving hospitals.