Too many people crowded into the central pen of the Leppings Lane End, whilst the stand may have been under capacity, that central pen was highly over. The polices failure to shut the tunnel off when it got too crowded was a contributing factor as well as opening the gate.
We all know the real root cause is football hooliganism of the late 1970's and 1980's though, all events transpired after this and Hillsborough was in fact an accident waiting to happen. As someone up the threat alluded to, that could well have been the United/Latics semi final a year later.
Slightly off topic, me and my mate were talking about it the other day, he said he read that Ken Bates wanted the fences to be electrified, in order to prevent hooliganism and pitch invasions... ridiculous!