Pyroclastic flows are usually the result of centuries of pent-up stress within the volcano; thick, sticky magma plugs the vent of the volcano and thus builds greater and greater pressure until it explodes.
Usually the flow is denser than the surrounding air and instead of flowing vertically, they rush along the ground, sweeping over hills and ridges like the said hills and ridges don't even exist.
Pyroclastic flows can reach temperatures as high as 1830 degrees F, and the average temperature is about half that; hot enough to boil the blood in one's body and effectively flash-cook you alive. Some victims are charred while others get fatal steam burns.
In short, a far more forceful and dangerous force than lava and more difficult to escape, due to their speed and intense heat.
May 8, 1902, about 30,000 people were killed in St. Pierre, Martinique, when Mt. Pelee sent a pyroclastic flow over the city.
Mount St. Helens famously erupted with a pyroclastic flow turning the surrounding forest into a lunar landscape.
The fringes of the pyroclastic flow do not always cut a clear line between life and death; survivors at the fringes often endure hideous burns.