Japanese hikers were part-way up Mount Ontake when it burped. One of the hikers took this footage of the ash cloud slowly rolling toward them like something from a terrifying monster movie. Then the cloud overtakes the hikers.
I don't think this is quite the same as a pyroclastic flow — otherwise the videographer most likely wouldn't have been posting this footage. But someone with some real geological knowledge can chime in on that.
It is a pyroclastic flow, as Mika McKinnon discussed over on the Space blog — give that a read if you want the real details of this event. The hikers in the video were fortunate that there wasn't enough of the noxious stuff in that flow to end them.