This is what happened to me yesterday - I was charting Btran cluster for exploration daily, and got into a random space mission to fight Undine ships. So I fought them, but once during a fight suddenly federation vanguard battleship appeared out of nowhere and helped me defeat the Undine vessel. After the Undine ship has been destroyed, federation battleship disappeared. I did see something like this before in Borg Hunt area, but dont know at all what is the trigger for it or how does this happen. It would be quite usefull in PvP I guess if it isnt just random chance for them to appear.

The ships helping you out is a scripted event. If you read the messages from your bridge officers/ship-to-ship communication it explains why they assisted you.