As I've heard tell, this was a glacial collapse into and blocking a river, which then built up a head of pressure behind it, bursting to release a hammer blow of river water and debris that travelled a great distance.

Similar things can happen due to landslides into rivers; it depends on the topology of the land, here long narrow valleys carved out by heavy water flows.

The warning being, don't live in a natural "drainpipe".

In mountainous regions that's often not much of an option, there's a lot of vertical sides and valley bottoms.