What causes slab pull in Earth's crust?

One plate is heavier than the other one at the subduction boundary. the heavier plates subducts beneath the less dense plate. The edge of the subducting plate keeps sinking (because it is much heavier and colder than the mantle) and pulls the rest of the plate with it. This is called slab pull.

Slab pull occurs when an oceanic plate subducts into the underlying mantle