What causes food aversions in pregnancy?

Pregnancy hormones. Two hormones, estrogen and Progesterone are quite elevated in a normal pregnancy. Both are likely responsible for the nausea and vomiting (morning sickness) experienced in the early part of pregnancy. As the pregnancy progresses, the nausea gets better but most women continue to find that even the thought of certain foods makes them feel ill. Fortunately, things go back to normal after birth.
Hormonal flux. Estrogen and Progesterone are in a major upheaval during pregnancy; wide ranging effects, including cravings and aversions.