A good few years ago a paper was produced which claimed to illustrate why standing up to fight would give an advantage to a male with these characteristics when it came to sexual selection (D CARRIER 2001). It was suggested that this led to a commitment to bipedalism .
To my mind the paper demonstrated why a biped is at a huge DISADVANTAGE in a fight with a quadruped of the same weight .

If you look at the paper ( see link below ) ,which is open access , focus on figure 2 . Look at the target height for each of the two strike conditions , quadruped and biped . The standing target is raised to above the height of any attacking quadruped .
Although the standing position allows greater strike power to a lifted target it does not allow that strike power to be transmitted to a lower ( quadruped ) target . A standing position also makes it much harder to bite an opponent but makes it far easier for a quadruped opponent to inflict its own bites .

So great is the disadvantage , when it comes to fighting , that a quadruped ape could never evolve into a biped without an evolutionary excursion into the tree tops and brachiation .

Therefore , out go theories like seeing over grass ,radiation and overheating brains , food collection , standing to feed , etc . Terrestrial quadruped to biped ? Not a hope .

The Advantage of Standing Up to Fight and the Evolution of Habitual ...
by DR Carrier - ‎2011 - ‎Cited by 44 - ‎Related articles
18 May 2011 - These observations, plus the findings of this study, suggest that sexual selection contributed to the evolution of habitual bipedalism in hominins.