OK, I found the Mine - a June 2014 aerial view from Google Earth here:
N 37 53 50
W107 38 38
Looking at the 2014 and previous images, I think I have a good idea what happened.
In 2006, the mine portal was producing no effluent - the mine pool had not yet risen to the portal elevation.
By 2011 the mine pool had reached this portal and a large volume of AMD was flowing out. But no effluent was coming from the possibly hydraulically-connected Mogul Mine portal a mile to the north and 400 feet higher in elevation.
On June 27, 2014, dirt was placed or slid over the portal and what looks like a corrugated arch pipe can be seen set at the point where the portal was located - but no outflow is visible from the pipe. Instead a smaller effluent flow is visible from lower down at the toe of the embankment. Also, tellingly, outflow has started at the Mogul portal, 400 feet higher in elevation. This doesn’t look good. Presumably the portal caved or there was a hillside slip plugging the portal where the pipe had been set and there now might be 400 feet of water head backed up behind the plug.
A major release was probably imminent at any time after late 2014, and the only way to have prevented it would have been to drill holes into the mine from higher up the mountainside and installing some large pumps and an extensive lime-treatment pond system - very difficult and expensive to do in this high country that is snowbound 2/3 of the year. It looks like the EPA was probably doing the best they could in a damned if they do and damned if they don’t situation.
I guess next time the EPA should remain aloof of all these environmental messes so that they will not be blamed when disaster happens.