Roger Westcott in 1967 proposed the "Upright Display Scenario" as one of the causes of bipedality, other causes include carrying tools and looking over tall grass while scavenging or predator avoidance. He suggests that bipedal behavior was adaptive because it made the bipedalist appear taller to intimidate their protagonists (other males) and predators. He also suggests that these displays were frequent and impressive. Westcott draws his scenario from threat display behaviors exhibit by the African apes; gorillas and chimpanzees.
Westcott proposed that early hominins had agonistic interactions which includes "two-legged standing or running, probably accompanied by fist-shaking or arm-waving, and possibly involving the seizing and brandishing of sticks or stones" (Parker and Jaffe, 2008).
Chimpanzee upright aggression. Photo from Why Files.Citation:
Parker S.T. Jaffe K.E. 2008. Darwin's Legacy: Scenarios in Human Evolution. Plymouth, United Kingdom: AltaMira Press.