Now this would have had me wondering had I seen it with my own eyes – a small cloud moves in strange ways, jumping from one position to another:
The explanation, provided by a meteorologist, has to do with the electric fields present within clouds (responsible for, most obviously, lightning):
The answer lies in this: ice crystals, especially long needles, tend to become aligned with the ambient electric field.
So what you are seeing is sunlight reflecting off ice crystal faces that are constantly being oriented by the developing electric field just above the [cumulonimbus] top. Then there is a discharge in the cloud, and the field collapses momentarily, and the crystals begin to realign again. Then this just keeps happening over and over.
Read more over at Bad Astronomy.