The same way 9/11 could have happened at any time?
Chernobyl was "a disaster waiting to happen" only in the same sense that the World Trade Center was. I mean, skyscrapers? Ooh, inherently deadly! But beyond that, it would take malice or idiocy to wring a disaster out of those, or out of Chernobyl, which was a relatively nicely and safely working atomic plant with plenty of failsafes to compensate for certain well-known risks in the design. Nothing could compensate for people who shut down failsafe after failsafe for the heck of it, though.
"Overmining" sounds like a very specific act of stupidity, but it's difficult to see how it would lead to an explosion as such. Work pressures and other indirect effects from lots and lots of mining would of course make an accident more likely to happen - but this tells us nothing about the nature of that accident. What could explode with such a force in a mine?
Well, any fictional substance X might blow up if hit the wrong way with a pickaxe or a mining phaser, I guess. But why would that be the result of "overmining" specifically? I do think some fairly complicated explanation is needed there.
(But no, my exploration of the alternate meaning of "mining" wasn't expected to be a serious explanation...)
Timo Saloniemi