Fluidized bed movement. In the reef hobby we use filtration that operates on this principle - in essence, instability with fluid movement of semi uniform particles allows those particles to move as if they were a liquid if that makes sense.

The base layer acts like a set of bearings mixed with lubricant, allowing the avalanche to roll a lot further than seems possible.

EDIT: liquefaction can have very strange effects.

Take the mass of a few cubic miles of rock, let it fall (slide down an underwater slope) a mile-ish, and there's an ample supply of 0.5mv^2.

And, whether above or below water, really huge avalanches have dynamics that seem quite counterintuitive. (Due to the square/cube law and other effects.)

Maybe via:

https://en.wikipedia.org/wiki/Turbidity_current

https://en.wikipedia.org/wiki/Submarine_canyon

If you look at google maps on ocean floor near any major river you'll see it continue for hundreds of km. Longest is 2400km in bay of bengal.