I'm betting on tool use for primates. Feet make lousy hands (and hands make lousy feet). Looking at primates that are semi-bipedal helps us seem some of the costs and benefits of bipedalism. Other animals use some bipedal travel, though. Kangaroos sometimes use 4 limbs and a tail to move, but other times use just their powerful back legs. Having small arms reduces weight. There are lizards who also "go bipedal" when they really run fast. We don't usually think about birds when we think about bipedalism, but clearly they are bipedal because feet make lousy wings.

I did some search on the web and found some useful article:

Big mystery: What made us stand tall? Researchers debate the possible factors behind walking upright. Carol Ward is an associate professor in the department of anthropology and the department of pathology and anatomical sciences at the University of Missouri at Columbia. She says at the very end of her article, "All in all, it's really an interesting puzzle and one that's not easily solved. The theories are, for the most part, based on logical arguments and everyone, of course, thinks their argument is the most logical."

Well, that makes me think that a variety of scientists (in particular anthropologists) have some pretty good ideas, but that they don't know for sure yet! Sometimes in science, there are several very logical sounding "potential answers" for the same questions, but there just isn't enough indisputable evidence for any one at the time. If this is the case, all hypotheses are equally plausible, and scientists must not arrive at any conclusions but ponder each hypothesis equally.

Scientists try to design tests to try to eliminate some of the potential answers. This is the stage anthropologists are at in deciding what was the driving force that led to the evolution of a standing posture.