Was it ever stated who or what drove the splinter of Dagon towards Avalos?
Final Sanction/Oblivion's Edge Followup?
Possibly the Symbiarch, as they can grow cappable of calling the hive fleet. But I don't know of any FFG released official followup.
Perhaps a radical Inquisitor, or one obsessed with Tyranids such as Ezu Ghraile (Rites of Battle p239).