“Whiny” is a term we apply to people who habitually complain about things that don’t matter. If people are complaining about things that do matter, then we generally don’t think of “whiny” as an appropriate term to describe them.
Social justice movements tend to spring up around issues that most people don’t get. Social justice movements tend to spring up around issues that, to most people, don’t seem to matter that much. If people understood that the issues mattered, then organized movements to promote them wouldn’t be necessary.
Until their issues are properly understood, most social justice movements, almost by definition, are going to look whiny to most people. If you can’t understand why the things people are complaining about matter, those people are going to look whiny to you. That is, they’re going to look like they’re complaining about things that don’t matter.
Something to keep in mind when you’re thinking about accusing people in a social justice movement of being whiny: every social justice movement looks whiny if you don’t understand their issues. A lot of the time, the fact that calling attention to their issues is perceived as whiny is precisely the reason why the movement is necessary in the first place.