There are certain species of monkey and ape that never really made it down from the tree tops which has also kind of been suggested as a possible reason to why they might not have actually progressed beyond a certain evolutionary point. That and a limited scope of exploration due to certain obstructive geographical factors that apparently help to limit their territorial spread and propagation as a species. 

Maybe it was coincidental that the type of monkey that eventually became Homo-Erectus and eventually Homo-Sapien (said without even the hint of a snicker) were generally land dwelling creatures and evidently exhibited a propensity to explore their surroundings to eventually evolve and populate the earth en mass. But it couldn't have been their strong tendencies to want to explore alone that that had helped Homo Sapien to eventually walk upright on two feet. Before you get all uppity Homo-Sapien is the modern day human. 

Maybe at some point certain behavioral traits and patterns became prevalent to become transmitted genetically and in turn socially. Our modern times would normally site the reasons in the opposite order in the belief that we have a greater mastery of our more primal instincts. Whatever the case I say it in that order as it would be naive to assume these creatures possessed advance active insight and complex consciousness from the outset for obvious reasons whether you ascribe to Darwinism or not.  But having said that Darwin would generally lend a fair weight of his outlook to my own personal stance on the matter. Maybe this is even a go between to suggest social factors along side the genetic played their part in shaping human ability to walk on two legs. It would still hold true to a large extent even if there were certain unaccounted for anomalous factors that haven't been included within our conventional understanding of how humans beings evolved.  

How I and some others reckoned it might have happened…

There could have been several reasons for this whether exclusively or in any number of combination whilst drawing on my ancient powers of insight if not actual memory. It might sound strange that something that was symbolic of evolutionary advancement could have potentially been derived from something so primal in terms of behavior, however there would have been the obvious disputes and territorial behavior between rivals within the same species if not other species and predators in the animal survival sense. Being able to initially stand up on two legs for short periods at a time could have served as a means of appearing more intimidating to potentially ward off predators and challenging opponents. The visual illusion of looking bigger. This would have also meant that those that were able to demonstrate such tendencies were also most likely able to exercise a greater degree of dominance in terms of mating which in turn maybe also helped to propagate the genetic trait further amidst the species given the herd like mating patterns most likely exercised at the time.

This in turn could have also co-incidentally helped to further ensure the survival (if only by slights) of those that were more likely to stand since it would have also acted as a means of allowing those capable to spot potential predators at greater distances amidst the undergrowth leaving those that weren't inclined to do such to just more generally die out from being more vulnerable to such threats. This would be especially advantageous where there were no large rocks to climb onto.

It could have also potentially become part of a mating ritual, a bit like a peacock might, but instead of putting a fanned out plumage on display they would stand up to demonstrate some sort of prowess for the females to gravitate towards the males that could demonstrate this ability thus further ensuring the genetic trait and tendency was further propagated in the natural selection sense but within a context of the beginnings of some kind of social construct.

However this in itself does not entirely account for the greater degree of ability to draw upon the cognitive ability and insight demonstrated within Homo-Sapien. Evidently there must have been some kind of factor that was missing from numerous tree dwelling (and none tree dwelling alike) primate species that didn't allow for their evolutionary development to progress beyond a certain point despite the fact that they were clearly able to propagate their species in a natural setting and survive to this present day . Or at least up until the point conservationists in the modern world stepped in to help ensure they didn't become extinct at the hand of man.