The nature of the slope materials:
Slope materials may be solid masses of bedrock, regolith (the surface debris, including soil, formed by weathering), or sediment. These slope materials may be unconsolidated (loose and uncemented) or consolidated (compacted and  bound together by mineral cements).
In addition,
The geological structure of the slope plays a vital role in the generation of landslides.
The steepness and instability of slopes:  The steepness and instability of slopes contribute to the tendency of materials to fall, slide, or flow under various conditions.