As I read, a picture formed. Loaded rolling contact between curved surfaces generates subsurface shear stresses. After many cycles of such stress, defects in the material may nucleate cracks that propagate through it. Such a crack typically reaches the surface of a part—a needle roller in this case, 3mm in diameter and 10.8mm long—at an angle. When it does, the sharp tip of the crack breaks off, creating a class-A surface pit. As the population of such surfacing cracks increases, two or more may join together to form a class-B pit. And now, the final stage of failure: When among these pits one forms of sufficient size, the particle of material released from the breakage of its tip is big enough to jam the roller, forcing it to slide rather than roll.