Sometimes that is just how NEC happens. Please remember the damage to the gut could be the result of an anoxic or hypoxic episode the baby underwent before, during or after birth. And really, the "experts" will tell you no one knows exactly what causes some babies to get NEC.
Speaking from experience, NEC does happen in full term babies, though thankfully it is rare. Usually their first presenting symptom is abdominal distention. Being a 30 weeker, there may have been subtle clues of illness before the baby showed abdominal distention. The classic precipitating event that I've noticed with preterm NEC babies is apnea. I start to get very nervous if I have a feeder/grower who starts to have apneic episodes where before their were none. Or, they have had a few, but the episodes become more frequent or require an increase in stimulation. Granted, this does not always mean NEC. But it could. Fortunately, our doctors and NNPs will take a change like this seriously enough to at least get serial CRPs to check for infection.