They got greedy and failed to remove overburden resulting in unstable slopes and eventual failure. As far as the price of copper is concerned, that pit is a shadow of its former output when it was high grade low hanging fruit, and realistically shouldn’t affect copper prices or availability (in a world without speculators).
I have been to a few mines of this nature. The walls of the pit are cut back in “benched” terrace excavation cuts to allow travel along the terraces as the back the cut wider and wider.
The underlying earth strata doesn’t always stay in place if ground water or other factors change. The banks of the entire pit have given way. The waste or tailings are placed elsewhere, not above the banks of the excavation as the overburden weight would be massive.
The earth strata is made up of layers or rock, shale, clays, silts and other materials. Generally they engineer what “angle of repose” is natural within the strata and design the benching to be as flat as the natural angle of repose for the native rock and soils. Sometimes they make a big error or the unanticipated happens.