Why do the pigs start walking upright?

In Animal Farm, the pigs start walking upright in the final chapter of the story and there are a couple of possible explanations for this. Firstly, the pigs have devoted a lot of time and effort to emulate humans. We have seen this earlier in the story when they, for example, start sleeping in beds, wearing clothes, and drinking alcohol. Although they hated their human masters, the pigs regard humans as the pinnacle of authority and power, so it is, perhaps, not surprising that the pigs would want to emulate this ability, even if it is very ironic.
Secondly, walking upright is symbolic of the pigs' elevated status above the other animals. Like their better rations and ability to change the rules, walking upright demonstrates that the pigs are not like the other animals on the farm. They represent a different class—the ruling class—and, by walking upright, this point is reinforced to everyone.