Ever heard of Yeast Autolysis
Autolysis is the process of self-digestion of the body content of a cell bits own enzymes. The slow disintegration and breakdown of the membrane of yeast cells, in the fermentation medium, allows for the passage of nitrogen into the beer, mead or wine. As the yeast cell dies, it ruptures, releasing several off flavors into the product including sulfur and rubber.
Like Pompeiisneaks said, when left without food the yeast start going dormant, but they can only survive when they're not active. Chilling helps prevent them from destroying their own reserves.
