I Really Don't Know Clouds At All

Amazing video of a bizarre, twisting, dancing cloud

The outer surface of ice crystals can hold a static electric charge, similar to what happens when you rub a glass rod with a cloth, or rub a balloon on your hair and stick it to the wall. When placed in an electric field, the charges feel a force on them, and align themselves along the field. So all the ice crystals above that cloud top are aligned one way in the field. Then the field snaps (maybe due to lightning releasing the energy) and then reforms. The ice crystals change their orientation suddenly when that happens.