Why is uranium hydride described as pyrophoric?

Uranium hydride ($U{H}_3$) is described as pyrophoric because it is a substance that ignites spontaneously upon exposure to air.12

This property is characteristic of many metallic hydrides, particularly those formed by inner transition metals like the actinoids. Uranium hydride is typically prepared as a finely divided black powder by reacting uranium with hydrogen gas at 300 °C (570 °F). Its high surface area and chemical nature make it highly reactive, leading to spontaneous combustion when it comes into contact with oxygen in the atmosphere.12