Cliffs collapse for a number of reasons, one of the most common being the effects of weathering, but there other factors as well, such as water crashing against the cliff face, what the cliff is made out of and the climate of the area. These factors can also work in conjunction to cause a cliff to collapse.
The weather can naturally erode the foundations of a cliff, causing it to collapse. One of the most common examples of weathering is when rain fall affects the composition of the cliff. As rain falls, it seeps into the permeable gravel and soil, adding weight to the cliff. At the same time, colder winter temperatures can cause the face of this cliff to freeze. When these frozen areas thaw, they are weakened.
Furthermore, if a cliff is against a body of water, waves can crash against the cliff, weakening it in time. Water can also naturally undercut a cliff by slowly eroding layers. With the weight of rain water on top and the lack of foundation caused by erosion at the bottom, a cliff can easily detach and collapse. Cliffs can also collapse as the result of rock slides, which is when whole slabs of rock detach from an inclined face and collapse. Cliffs also collapse as a result of mud slides when saturated soil and weak rock fall, creating what's called a solifluction lobe.