The downhill movement of soil and loose unconsolidated sediments is due to the force of gravity and is resisted by the force of friction. The forces of gravity and friction are in balance at the angle of repose which is the maximum slope angle that unconsolidated materials can maintain. At angles steeper than the angle of repose friction is not sufficient to counter gravity and mass wasting occurs. At angles less than the angle of repose gravity cannot overcome friction and sediments may accumulate to form steeper slopes.
Water plays an important role in mass wasting. Dry sediments have no cohesion. Damp sediments are cohesive as water coats the sedimentary grains and holds them together with its surface tension (surface tension is the result of the dipolar nature of water). The angle of repose increases (think of a sand castle). In water saturated sediments all the pore spaces are filled with water. Water pressure and water's buoyancy forces reduce the friction between the grains thereby decreasing the angle of repose and possibly causing mass wasting.
The two basic classes of mass wasting are flows and slides. In flows, the material behaves as a fluid. Soil creep, earthflows, and mudflows are examples. In slides, the material behaves as a rigid solid that detaches along a basal surface. Slumps and landslides are examples.
Steady soaking rains, undercutting slopes for road building and house sites, and removal of vegetation by fires, etc may induce mass wasting.