We haven't heard much from the stranger since vanilla D1, except for some classic destiny ARG shenanigans and a few hints to her identity lodged in sparrow lore. But now that black garden vex are spilling onto the moon, it would be a perfect time for her reemergence. Her ability to time travel may mean that she already knows the outcome of everything to come, and has decided that showing up on the moon wouldn't actually serve a purpose. But in D1 she seemed to have a vested interest in our guardian, and the vex, so it makes sense she would appear again. (If there's any lore junkies that actually know what shes up to now, please let me know)