Here's why:

* Speed: Rockfalls happen quickly, often with rocks and debris tumbling down a slope at high speeds.

* Movement: The material involved (rock) moves rapidly in a falling, bouncing, or rolling manner.

* Mass: It involves a significant amount of material, from small pebbles to large boulders.

Other examples of fast mass movement include:

* Debris flow: A rapid flow of mud, rocks, and water, often triggered by heavy rainfall or snowmelt.

* Landslide: A rapid downhill movement of soil, rock, or debris, usually triggered by earthquakes or heavy rainfall.

* Avalanche: A rapid flow of snow and ice down a mountainside, often triggered by heavy snowfall or vibrations.

These are all examples of fast mass movement, characterized by rapid, downhill movement of significant amounts of material.