What are all the reasons for land slide in hills area ?

The reason for landslides in hill areas is because of lack of friction between soil grains.

This is a combination slope angle, load, and often water content of soil that will lubricate grains allowing slipage on an unstable slope. Soil type is also a contributing factor.

Essentially a slope will fail resulting in a landslide when friction forces are exceded by gravity. This often occurs in areas where a forest fire has occured because the burning of plants creates a waxy coating on soil several inches below the surface so that water pools at the surface increasing the load at the surface.

The best way to monitor a slow moving landslide is with an inclinometer. To monitor slopes that could potentially fail requires constant observation for many factors by a geotechnical engineer. There's no one way to know however conditions such as forest fires, as mentioned above, as well as others increase the potentiality.

Geologists should be able to take soil samples and see if it the type that will stay stable with excess moisture.

Water flows underground and some soils loose their integrity and become mud.

This is most dangerous on hill sides.