Science lesson of the day.

Clouds can carry huge electric potentials — essentially, the ability to move charges around — and that stored energy can be suddenly released, creating lightning. When that happens, the electric field resets itself, and starts to store up energy again.

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.