As you build an earthen dam, you need to make sure that water can move through that structure. If you try to barricade that entire dam, you risk its becoming saturated and risk a more catastrophic failure. What you do, then, is allow rainwater, runoff and those types of things to move through it so that you have a containment area that involves a number of different strategies to intercept any of that seepage and move it back into the tailings facility.
What occurred in this case is that the containment system in four different locations failed to contain that seepage, and it moved through a specific layer.
There are updates on our website if you want to see if there is more detail about what's happening.