Answer: Pryoclastic flows happen if a lava dome collapses, the lava shoots outwards, but collapses and if a volcano is damaged.
Explanation: If a lava dome collapses, it can cause a pryoclastic flow. During a eruption, it may snap off if it explodes. Like the eruption of Mount st. Helens. It swelled up then later the peak snapped off, creating a landslice and pryoclastic flow.