Causes
One of the most common causes of landslides, it applies extra weight onto the top layer, and the resulting mud can reduce friction, which plus the extra weight, can cause the upper layer to slide away.
Yes, deforestation can result in more than just lost homes for animals, for roots from trees can help hold land in place, and if the tree is dead, the roots will rot and not help hold the land in place, the land can have a danger to slip.
Earthquakes can shake layers of the ground loose, which makes the ground susceptible to landslides.
Gravity can push down on weakened parts of the land, and slowly make it lose its grip on the rest of the land, causing a landslide.
Things to Prevent them
You can drill large bolts deep into the ground to hold the upper layer in place.
If you can make the slope of a cliff in a less straight direction, then it can prevent landslides.