Well, the Lurks are vampires, and vampires are immortal, so maybe a few somehow managed to survive the purge that was the result of the final battle mentioned in the "Fray" series. If that to be is indeed the case, assume, also, that Dark Willow, who has apparently managed to restore magic, at least for her own personal use, somehow activated one or more of those surviving vampires' ability to sire new vampires. Further, accepting all of that, assume also that the increase of actual vampire population triggered whatever Called Fray. Remember what she was told. Slayers had continued to be born, but were never Called, probably because there was no need for them anymore. Once there became a need once more, the Slayer cycle resumed.
As for a likely candidate for the "surviving vampire," I would suggest that Angel, being one of the good guys, would have been exempt from the purge. Centuries pass, Angel is living the life of a recluse somewhere, maybe still brooding about loving a mortal woman and having to watch her grow old and die with nothing that he can do about it. Dark Willow finds him and removes his soul, this time permanently (or even better, transfers it into the body of a four armed monkey) and Angelus begins raising a vampire army.
Further, maybe Spike managed to survive as well, and upon discovering what Angelus is doing, tries to stop him, resulting in the two vampires dusting each other, which would explain why neither one is present in the actual "Fray" series.
(Joss, if you're reading this, the same deal as usual applies. feel free to use all or any part of the above, with no acknowledgement to me either required or expected)
