The problem in disused mines, in limestone strata, has already been described by historytrog.
Basically the limestone, has clay layers, in it. These hold pieces of the roof in place, and can, and do from time to time, detach themselves.
Most of the time these falls, take place when nobody is around, however, in this tragic, event, unfortunately some one was.
It all reinforces what we all know, exploring disused mines can be dangerous.