These things happen way too often. The famous Johnstown flood and the Buffalo Creek mine disaster are two similar issues that come to mind but there have been others.
I don't know why they allow these earthen dam impoundments and allow them to continue existing after they are no longer needed. Corporate greed and poor regulatory oversight I guess.
Ohio is one place that at least appears to have done a very good job of reclaiming strip mine land following the closing of the mines.