As with most incidents that end up with large numbers of casualties there isn't one thing that can be blamed. More a tragic coming together of a lot of a lot of little things at the same time. On their own it wouldn't have caused such devastation. But put them all together and the worst happens. So, yes, fans acting like they did on the day were a little piece in a terrible jigsaw. On it's own it wouldn't have been enough. But tied in with all the other failures that day and what happened, happened.
There is no one cause for Hillsborough. But lots of little things that all came together at the wrong time.
What is unforgivable though was the way the authorities and some sections of the media acted in the aftermath