The most obvious point to draw is that this is the furthest along the Doctor’s timeline that she [River] will have been, so it changes exposure of key events that she could be aware of. Of these the most obvious is that she would become aware of the fact that the Doctor changed his own personal timeline in order to save Gallifrey instead of destroying it. I would hazard a guess that the reason for River’s return could be linked with the Doctor’s search for Gallifrey. The search is likely to pick up pace by the time Christmas comes around, so it is entirely plausible that would mean that River could be involved.
Mark McCullough, DoctorWho TV - Speculating On River’s Return [x]