According to the Daily Mail, research implies that about eight million years ago, supernovas triggered lightning strikes that spurred ancient primates to stand upright. Forest fires happened, so our arboreal ancestors had to scuttle from tree to tree instead of climbing. I don’t know about other people, but that sort of thing would alarm me a bit — I’d want to stay low to the ground!
Still, there are other, more reliable sources of information than the Mail reporting the same thing. Phys.org says:
“It is thought there was already some tendency for hominins to walk on two legs, even before this event,” said lead author Adrian Melott, professor emeritus of physics & astronomy at the University of Kansas. “But they were mainly adapted for climbing around in trees. After this conversion to savanna, they would much more often have to walk from one tree to another across the grassland, and so they become better at walking upright. They could see over the tops of grass and watch for predators. It’s thought this conversion to savanna contributed to bipedalism as it became more and more dominant in human ancestors.”