The dark dog arises from an instability in the cloud. The details and what triggers it are uncertain but the broader picture is known.
Water does not always freeze at 0 degrees Celsius. If no nuclei are present on which ice crystals can grow (heterogeneous nucleation) the water can remain at sub-zero temperatures - it is supercooled. At sufficiently low temperatures spontaneous nucleation occurs but at intermediate temperatures clouds can be composed largely of tiny supercooled water droplets.
Thermodynamically this is a metastable condition because ice is the more stable state. Introduce enough nuclei or otherwise disturb the cloud and ice crystals grow very rapidly.
This happens because the equilibrium vapour pressure of water over supercooled droplets is greater than that over ice at the same temperature and because ice crystallisation releases heat. Once ice crystallisation starts it continues rapidly with the necessary water vapour supplied by evaporation of the surrounding water drops. Very soon we have a cloud largely of ice crystals rather than one of supercooled water drops.
This is not special. It is one of the major mechanisms by which rain forms. The water drops in clouds are not usually able to collide and coalesce sufficiently to form large raindrops. Ice crystals can, however, easily grow and cluster into snowflakes. The cloud then snows, or if the temperatures are high enough, the flakes melt into raindrops.
What is unusual is that a relatively small (several hundred metres) region of a cloud should suddenly transform from supercooled droplets into large halo forming ice crystals leaving a hole.
Several hypotheses exist to explain the necessary localised disturbance. Falling ice crystals from higher cloud fragments might initiate the process. Disturbances by aircraft have been suggested. Natural high and medium level turbulence and air currents might also disturb the delicate metastable state.
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