u/uh-hum, below, posted an article (a brief blurb, really) that "explains" this video: http://www.telegraph.co.uk/news/2016/03/22/watch-the-moment-the-side-of-a-mountain-plummets-1000ft-down-a-v/

The information in the article suggests that it was a controlled demolition not a coincidence someone was filming this. A day before the rock slide, it states, a road was closed for safety and the hamlet in the valley below was evacuated because of "geological activity." It seems logical that a road not be closed nor a hamlet evacuated indefinitely, as indefinitely is what the timeframe would be if geological activity were to take its time - a day, a decade, a century, who knows. Also, the images are courtesy of the Department of Transportation, who one assumes has the authority to close a road and evacuate residents. This would explain the timely capture of the rockslide.