Like some terrifying land version of a tidal wave, pyroclastic flows happen when an ash cloud collapses and in a glowing avalanche sweeps down the volcano slopes, causing damage not just by burial and incineration but also by impact damage and asphyxiation.
In a Pelean eruption, glowing avalanches made of dense, hot gases are present along with the large amount of gas from the magma that causes pyroclastic materials.