Muscle fatigue - The system that initiates muscle contractions is increased (hyper-excitement of muscle spindles) while the part of the system that inhibits contractions is decreased (Golgi tendon organ). This would result in cramping only to the muscles that were fatigued. In a case of cramping from dehydration, you would notice cramping throughout the body, and not just in one specific muscle.
Symptoms of common muscle cramps
What can make an athlete more susceptible to cramping?