Water can build up in the ground and add additional weight and lubrication. If underlying ground is impermeable, then the layers above may be free to slump or slide. The higher the gradient and the more porous the top material is, the more unstable the situation will become.

Water picks up velocity by moving swiftly downhill and energy by picking up sediments. Thus in a desert environment, flash flooding is quite dangerous even on relatively flat land. Debris filled water may rapidly move down steep canyons and then fill relatively shallow stream beds at high speed with added mass.

Volcanic Action
Pyroclastic materials near the top of stratovolcanoes are often right at the edge of their stability limit. They have achieved their natural stable gradient.

When a large volcano warms up even before eruption it may have sufficient energy to melt snow or glaciers on its side. The added water run-off is often sufficient to destabilize the steep gradient debris near the top and bring it down.

Unconsolidated ash from the current eruption or a previous eruption can be easily mobilized by new volcanic activity. Mud flows, or debris flows created by this mechanism are called Lahars.

Underwater Failures
Earthquakes work well here too, and these quakes may be deep sea based and not obvious to ground based observers.

Water pressure created by overlying sediment. As sediment builds it buries older sediment and the added pressure may squeeze water form the older sediment and create a rapid water flow.

Methane gas release from the ocean floor.