Typically what is required is a long saturation period followed by intense bursts of rain concentrated in just a few hours or days. Water passes rapidly through surface material until it hits bedrock or clay, creating a saturated zone as it fills the pores between solid particles. Rising pore pressure creates a buoyancy that effectively reduces the stabilizing friction of the colluvium on the slope beneath it. At some point gravity overcomes the natural inertia, and the soil mass breaks loose, sliding down the less permeable surface below. Or maybe not.
Water can collapse a slope after traveling beneath the surface from miles away.
Vibrations from trains are suspected of triggering debris flows. And sometimes there seems to be no particular cause. In September of 1997 a man sitting at a bar in Port Angeles, Washington, was crushed to death when a mudslide destroyed the building. The area had had no appreciable rain for months. Large, deep-seated slides, in which the dislocation originates far below the face of a slope, tend to occur more slowly than shallow debris flows, which are more deadly.