What reasons are there for doing an early delivery at 34-36 weeks?

Risk. Any risk to your fetus or your own health would compel the ob/gyn to do an early delivery.
Complications. Conditions that may make early delivery necessary are varied. Problems with the placenta like a previa, accreta or abruption won't go until term. Previous uterine surgery affecting the main muscle part of the uterus needs to be delivered early. Low fluid or broken bag of waters, triplets or certain twins all may go early. If the mother has bad diabetes or high blood pressure, early may be better.