So the root thing behind this that we only relatively recently figured out is that much of it comes from us having value based identities (as opposed to organization/association based identities).

Allistics tend to have much more fluid relationships to their values, because while important, their values aren't fundamentally who they are. Instead their identities are based off of their social structures (think things like "team spirit", "hometown pride", etc). Their values shift based on whether people are watching and how their actions relate to the groups they associate themselves with.

We on the other hand typically root our value purely in our values: the choices and actions we make. We're consistent regardless of whether someone is watching, as well as whether or not the thing impacts groups we're a part of. (things like "If it can be destroyed by the truth then it should be")

So of course we're angered by injustice... it's people and groups around us betraying their own values constantly. And so often one of our key values is "stand up against oppression", so we'd be betraying ourselves if we didn't get infuriated.