More pics of the landslip :
It looks to me as if a huge rock split and half of it fell away, perhaps after the snowfall? Speculation: Could be that the freezing temperatures split the giant rock, then heavy rain shifted muck at the base that was supporting one half of it on the hillside and then it rolled away. It looks from the pictures of the half that remains, as if there was a vertical-plane (ish) crack, or one at an even more unstable inclination, and the crack very approximately ran from left to right as you look up at it from below, so just right for half to fall away.
You get an idea of how huge it was, as the pylon looks tiny. They said 9000 tons of stuff had come down.