The storm system, which occurred during the start of Saturn spring, grew in size and intensity, eventually stretching around the planet. Scientists don’t exactly know what stirs up the storms, but they think it could be linked to the change of seasons.The Associated Press article, which I've taken from the CBS Sacramento site, also has a link to a recording of the storm. That helps to answer an ancient riddle: "If a storm happens on Saturn and no one is there to hear it, does it make any sound?"
At the height of the storm, Cassini detected 10 lightning strikes per second. Scientists said the electrical activity emitted by the bursts were 10,000 times stronger than lightning on Earth.
The cited article also says that Jupiter is a hotbed of storms, while they're rare on Saturn.
(Thanks to Joe Case for tweeting this interesting story.)