Wrong question. The proto-dinosaurs were carnivorous and bipedal. It's a legacy thing. This question is the development of vegetarian diet and quadripedalism,
Wait a minute. If proto-dinosaurs were bipedal and carnivorous, what did they eat? There must have been some contemporaneous herbivorous proto-dinosaurs, no? Or did they they ate herbivors of a different order/phylum?
http://microlnx.com/dinosaurs/Bipedality.html