I think UCLA tries to reach out/informal-interview early (before Winter break), at least for the social department. So they probably have already reached out to the shortlisted candidates at this point.
I think this is school dependent? I heard (and I might be wrong) that Stanford admits students into their department as a whole (competing for a spot in the program), but there's also schools like UCLA where each professor is responsible for inviting students they'd like to admit to their lab, and they just pick from those that they shortlist/invite. Hope this clarifies things!