iirc, the beetle does have some animations that are unused, don't have the actor .blend with me to verify atm.
Looks (to me anyway) like the "stomping" you're seeing is the start of their animation to burrow back underground from samus leaving the area, but you're not quite far enough away to do this so it infinitely tries to burrow back underground. Again, failing to do so because you aren't "far enough" away to make them despawn. Just my opinion.