We know almost nothing about that Cylon fleet, or where Starbuck came from (other than Earth.) The only thing we know is that they didn't track the rag-tag fleet there by following the radiation signature of the now-decoy refinery ship.

Clearly some powerful forces have been planning something about that nebula for some time. Aside from leaving a 4,000 year old clue to point to it, the final 4 were set to trigger there. They were not triggered by something at the nebula itself, but by "proximity" to it of some sort. They started hearing the music hundreds of light years away from the nebula, which implies that they either had a hidden command to trigger when they became aware they were approaching the nebula, or there is some sort of non-FTL field that surrounds the nebula for a thousand light-years, or the being that sent the trigger is traveling with the fleet. Or they received an FTL trigger from somewhere distant.