Pyroclastic flows are fast currents of hot gases, rock, and volcanic ash that move rapidly in response to gravity. They move away from the vent that erupted them at extremely high speeds making them the most deadly of volcanic phenomena. The reason they are so deadly is due to the extreme speed with which they travel catches people unaware.
What makes these eruptions so powerful? You need to see how moisture and lava interact. For example, if you've ever used an iron or poured water on a heated stove the water would immediately vaporize with A LOT of noise. However, that is what happens with a solid heat source. The reaction is more powerful if you mix cold water or ice with lava. The cold water makes the lava cool rapidly making the ash and volcanic rock that is expelled in a pyroclastic flow also releasing the trapped gases that make such eruptions move so quickly.
One of the many reasons that pyroclastic flows kill people is because they contain toxic gases.