Sedimentary beds can exhibit signs of local deformation in pyroclastic strata. Patterns are reviewed and trigger mechanisms interpreted. During an eruption, basal granular flows can have a fluidized behavior, inducing over- or underpressure at the bed interface. Basal shear can overturn strata. Large blocks ejected ballistically deform the ground when landing. Explosions at the vent produce shock waves that can destabilize a bed. These syn-eruptive triggers are specific to explosive volcanism.