Atheism refers to the lack of god-belief, meaning that it does not require any more of someone than not believing in gods. In the United States, atheists are far more likely to vote for Democrats than they are to vote for Republicans; however, there is nothing inherent in the meaning of atheism that requires this to be the case. I suspect it is the case mostly because the Republican Party has explicitly embraced evangelical fundamentalist Christianity at least since the late 1970s. It is no secret that evangelical fundamentalist Christians regularly push things with which most atheists strongly disagree, so it makes sense that relatively few atheists would support a political party that aligns with them.
Briefly consider three issues on which the religious right (i.e., evangelical fundamentalist Christians who have aligned themselves with the Republican Party) has been rather vocal: women's rights (e.g., reproductive rights, gender equality) and LGBTQ rights (e.g., same-sex marriage, LGBT service in the military, discrimination by private businesses), and censorship (e.g., banning books, music, films, and other forms of creative expression deemed objectionable or blasphemous). Their rationale for the positions they take on these issues are nearly always couched in religious language.
In every case, it appears that their goal is to restrict others' freedom on the basis of their religious beliefs. I think it makes sense that most atheists would be opposed to this. Similarly, I think it makes sense that most atheists would be opposed to repeated expressions of anti-atheist bigotry or attempts to abolish church-state separation and replace it with a Christian theocracy. None of this is to say that atheists cannot vote for Republicans; it just seems like doing so requires them to overlook an awful lot of stuff that strikes many of us as being hard to overlook.
But what does this have to do with social justice? If you are an atheist who values the separation of church and state and is not interested in religious believers imposing their religious beliefs on you, you just might be inclined to support at least some social justice causes. I'm not saying this must be the case, but I would suggest that it is likely to be the case. I think this is part of the reason why we are seeing many of the national secular organizations adopt at least some social justice goals.
I think it is easy to understand why this is happening. When a secular organization sees a member of Congress using his religious beliefs as a rationale to promote anti-LGBTQ bigotry, restrict reproductive rights for women, dismantle reality-based sex education, promote religious schools at the expense of public schools, or attack climate science because "the end times" are upon us, they are likely to address it. And why shouldn't they? Opposing this sort of nonsense is one the primary reasons these organizations exist. Hasn't that always been the case?
I don't think we have any need for a national secular organization that does nothing but atheism. I'm not sure what that would even look like. At minimum, I think we should expect our national secular organizations to defend the separation of church and state, promote secularism, and work to eradicate anti-atheist bigotry and discrimination (and other forms of religiously-motivated bigotry and discrimination). Beyond that, I think we should expect these organizations to oppose many other efforts by the religious right to impose their religious beliefs on us. Specific areas of focus will likely differ from organization to organization, but unless I see them prioritizing things that do not appear to have anything to do with secularism or which do not involve religious motives of any kind, I'm inclined to welcome it. That is, I'd like to see more social justice advocacy rather than less.
Being an atheist has never been sufficient for me, and I expect it never will. There is far more to me than a lack of god-belief, and it is hard for me to imagine any atheist being truly content to be an atheist and nothing else. This does not mean that I think all atheists must be liberals, humanists, or involved in any social justice causes. But my suspicion is that most atheists who value secularism and are less than 100% satisfied with the status quo are positively disposed toward at least something that could probably be placed under the social justice umbrella.
