[rdc12]

Different volcano, but back in the 50's the natural tephra damn on the crater lake of Mount Ruapehu (New Zealand) collapsed triggering a lahar, that ultimately took out the piers of rail bridge near Tangiwai minutes before a passenger train tragically attempted to cross the bridge, resulting in mass casualty's.

So certainly possible for non-expulsive activities to trigger a lahar and for consequences that sound like they are more from a disaster movie then real life to occur.