  We explain the fast-moving, ripple-like features in the edge-on debris disk
orbiting the young M dwarf AU Mic. The bright features are clouds of sub-micron
dust repelled by the host star's wind. The clouds are produced by avalanches:
radial outflows of dust that gain exponentially more mass as they shatter
background disk particles in collisional chain reactions. The avalanches are
triggered from a region a few AU across -- the "avalanche zone" -- located on
AU Mic's primary "birth" ring, at a true distance of $\sim$35 AU from the star
but at a projected distance more than a factor of 10 smaller: the avalanche
zone sits directly along the line of sight to the star, on the side of the ring
nearest Earth, launching clouds that disk rotation sends wholly to the
southeast, as observed. The avalanche zone marks where the primary ring
intersects a secondary ring of debris left by the catastrophic disruption of a
progenitor up to Varuna in size, less than tens of thousands of years ago. Only
where the rings intersect are particle collisions sufficiently violent to spawn
the sub-micron dust needed to seed the avalanches. We show that this picture
works quantitatively, reproducing the masses, sizes, and velocities of the
observed escaping clouds. The Lorentz force exerted by the wind's magnetic
field, whose polarity reverses periodically according to the stellar magnetic
cycle, promises to explain the observed vertical undulations. The timescale
between avalanches, about 10 yr, might be set by time variability of the wind
mass-loss rate or, more speculatively, by some self-regulating limit cycle.
