Here's why:

* Gravity's Pull: Gravity constantly pulls everything towards the Earth's center. On slopes, this pull can be significant, especially for loose rocks or rock formations weakened by other agents.

* Triggering Factors: While gravity is the primary force, other agents can trigger a rock slide:

* Water: Rain, snowmelt, or groundwater can saturate the ground, making it unstable and prone to sliding.

* Earthquakes: Seismic activity can shake the ground, dislodging rocks and triggering landslides.

* Human Activity: Construction, deforestation, or blasting can destabilize slopes and increase the risk of rock slides.

So, while gravity is the main driver of rock slides, other factors can contribute to their occurrence.