A landslide may occur because the strength of the material is weakened. This reduces the power of the 'glue' that cements the rock or soil grains together. Located on a slope, the rock is then no longer strong enough to resist the forces of gravity acting upon it.
What can increase the chance of a landslide?
Several factors can increase a slopes susceptibility to a landslide event:
water (rainfall or the movement of the sea) — this acts as a grease to the material increasing the likelihood that it will slip and also adds extra weight to the rock
erosion processes — such as coastal erosion and river erosion
steepness of slope
type of 'rocks' — soft rock such as mudstone or hard rock such as limestone
shape of the rock 'grains'
jointing and orientation of bedding planes
arrangement of the rock layers
weathering processes — for example freeze-thaw reduces the stickiness (cohesion) between the rock grains.
lack of vegetation which would help bind material together