Why did natural selection favor bipedalism?
Scientists actually aren't decided on why natural selection favored bipedalism in humans, and there are many ideas.
There are multiple theories as to why humans walk upright. For example, some believe that we evolved to walk upright in order to see over tall grasses, although others argue that this would have immediately announced our presence to predators. Some believe we started walking upright because we were using stone tools, but the earliest stone tools show up in the fossil record long after our ancestors started walking upright.
Others argue that bipedalism is more efficient, meaning we use less energy, than walking on all fours over long distances. Indeed, a recent study showed that humans are about 75% more efficient than chimps when chimps walk on two legs due to differences in our anatomy.
Another theory states that walking upright was advantageous for males because they were able to carry food back to females with offspring. Males provisioned females and therefore lowered reproductive costs. There are problems with this theory too, as our primate relatives show it's usually the females who provision their young. So this idea of our female ancestors lying around burdened with offspring ignores what we see in living primates.
Here's one anthropologist, Dr. Donald Johanson's thoughts on the evolution of bipedalism: http://www.pbs.org/wgbh/nova/evolution/what-evidence-suggests.html
Here's another great resource on bipedalism from the Smithsonian: http://humanorigins.si.edu/human-characteristics/walking
The bottom line is that we still don't know!