Why did Gandalf let go and fall off the cliff?

[Manwe Sulimo]

Actually, I was once dragged to the edge of a cliff by a Balrog and had to run through a hail of arrows to save myself. Let me tell you, it hurts like Udûn.

....But even as it fell it swung its whip, and the thongs lashed and curled about the wizard's knees, dragging him to the brink. He staggered and fell, grasped vainly at the stone, and slid into the abyss. 'Fly, you fools!' he creid, and was gone.

He never "let go", he never even had been hanging on. He (quite clearly) fell right over.