I am a very fit cyclist who gets side cramps from time to time. I have been able to narrow down the cause to the recovery cycle following century rides. For several days following a century, during intense efforts, I get side cramps. I attribute these to a lack of fuel stores (I know there is a large one in the liver, an expert on the forum can expand). The cramps dissappear when I have fully recovered.
Lately, I have taken the recovery cycle much more seriously to head off this and other effects.