To create the growth activation effect you see in the animation, you slowly wake up the grains, but not all at once.

Polishing up the Grains Simulation Visual Effect. Terrain is made with Heightfields in Houdini and added a Vellum Grains simulation for rock slide effect.

Adding Vellum Grains Simulation to my Heightfield Terrain created in Houdini from the previous couple of posts.