I get the worst cramps climbing down the stairs at work. At that point, I'm not dehydrated, and have no electrolyte imbalance. I can only assume it's a form of muscle fatigue, from over-using muscles that aren't loosened up sufficiently to take the load.

A good starting point for the explanation: http://www.ultracycling.com/training/cramping.html

Good hydration and nutrition matter, of course, but the more I look into this, the more I like the muscle fatigue/over-use theories.