According to the Farmers Almanac...

Stood in a long line getting some groceries and started reading the 2011 Farmer's Almanac... According to an article in there the reason fish bite before a storm is that the low barometric pressure releases gas bubbles that cling to decaying matter at the bottom. The bubbles rise to the surface, carrying the decaying matter with them. Small fish follow the particles to feed, big fish then follow to feed on them.

I've never heard that explaination, not sure if it's true but thought it was kind of cool...