A normal lightning strike to the ground is the result of the attraction between the negatively charged cloud and the positive charge on the ground below. What causes lightning to strike from one negatively charged cloud to another?
We normally think of sparks jumping to an opposite charge, either negative to positive or positive to negative. A spark can also jump from a strong, negative charge to a weaker, negative charge. That is what often happens with cloud to cloud lightning. An instant before this shot, there was a large bolt from the cloud to the ground (or in this case, the ocean). That bolt removed quite a bit of the charge from the cloud, leaving it with a weak, negative charge. The surrounding clouds were strongly charged, letting sparks jump from them to the cloud with the weaker charge. This can sometimes cause chain reactions where the lighting seems to crawl across the sky.