Hard to tell. Maybe it just wasn't your day. Nothing that I've read seems to have a really good explanation for cramping. Personally I think any number of things can contribute, such as dehydration, electrolyte imbalance, even weather for instance. I've had calf cramping on a cool day with a lot of wind, so that the muscles had a hard time staying warm.. But recent articles that I've read suggest that conditioning (or lack thereof) is the biggest factor. Still, only a mile and a half into the race . . . maybe you just started too fast. Check this article; http://www.runnersworld.com/injury-treatment/why-do-my-calf-muscles-cramp-when-i-run
I had serious cramping a mile and a half into a frigid marathon, making it the only one I had to quit. Hours later after warming up, I was fine. Many people don't remember that muscles are the best heat source for the body, and to generate heat, they contract. Shivering is an example of repeated contractions, but increased tone alone can achieve the same effect. Neither bode well for the relaxed muscles you need to plow into sub-optimal temperatures in an athletic contest. Add nerves to that, and you had a perfect storm for cramping. You may never get your money back on the day of a race for bad weather, but you'll always get another chance to run more comfortably. Better luck next time!