As we’ve mentioned before, chimpanzees are very territorial and it’s within their nature to defend their home against strangers. In order to be intimidating, they will usually stand up on their legs, swagger, and their hair will stand on end (pilo-erect). Then, they usually bang something, vocalize, or otherwise make some sort of threatening noise. Threat displays are just that—displays. It’s all just to show how scary they could be, if you don’t watch out.
Sometimes, dominant chimpanzees want to show the others in their group that they could be pretty scary if they want to be. It helps solidify their role as the leader in the group—the intimidation aspect strengthens their control. A dominant chimpanzee will occasionally display their dominance with no provocation at all (or at least, nothing immediately observable).
Here’s a video of exactly that, a display just for the sake of displaying. It doesn’t mean that Jamie is angry or upset about anything, it’s just something she does to remind everyone (chimpanzees and humans) that she’s the boss around here.