I looked up diverse documents briefly, trying to find some association between FRC diving and cramps, and although I did not find any studies addressing cramps and FRC diving, or freediving in general, and not even direct association of vasoconstriction to cramps, some of the information is quite consistent with Connor's idea.
Some of the previous tips are more or less valid, and the lack of magnesium, or free calcium ions depletion (for example due to hyperventilation) can indeed contribute to the onset of cramps.
Then there are also number of medical reasons that can lead to muscle cramps - one of the most important is undiagnosed diabetes.
On the other hand, the opinions on the importance of hydration or sufficient salt level seem to have changed lately: the popular theory is that exercise-induced muscle cramping (EAMC) is caused by fluid imbalances, particularly dehydration and abnormalities in blood electrolyte levels, but there are recent studies denying it (some info for example here:
muscle cramps or here
No link between hydration, cramping? - Vox)
So according to the modern studies, the real reason for cramps is not really hydration or salt imbalance, but simply muscle fatigue. To avoid them, good muscle conditioning (training, stretching) is important. Cramps are more likely with older age, a higher body mass index, shorter daily stretching time or irregular stretching habits, and if there was a family history of cramping. Some more information also here:
cramp: Definition, Synonyms from Answers.com
Now, it looks like that despite the studies, the exact mechanism of cramps is not yet fully understood. I do not know what the "muscle fatigue", causing the cramp, exactly means from metabolic, or chemical point of view, but I'd tell that the influence of proper oxygenation is rather likely. Hence, it would not be surprising that at FRC diving, where your muscles are deprived of oxygen earlier than normally (due to vasoconstriction), they cramp easier.
So to prevent the cramps, good muscle conditioning and especially consistent and proper stretching before and after exercise are very important. I also found
this document speaking about using ribose for cramp prevention, but do not know what side-effects it would have at freediving (or generally), and also do not know if it is not a substance forbidden in competitive sport.