What happens during pregnancy that might cause gastrochesis in a baby?

Sporatic event. It may surprise you but we all go thru a stage in early embryonic development where our intestines are outside of our abdomen. Gastroschesis is thought to represent a random event during pregnancy where the gut does not follow the signals and move into the abdomen by the 10th week. I know of no association with any maternal action, medication or illness that would trigger the event.
Not sure. Many theories abound. The most common is that of a vascular accident or clotting of a vessel that feeds the tissues forming the abdominal wall. Without adequate blood supply the abdominal wall may be weak or not form at all.