There is only slightly more order to this world than Genessia. Regularity in the time in which new arrivals appear doesn't necessarily mean that there must be a specific cause. That said...

{ He sighs. }

The Queen of Darkness controls this castle, but can't stop the summons. Her power here is not absolute. If there is indeed a ritual... It is performed in a place neither she nor we can reach.

{ Zeref shook his head at that. }

Every time they seized more ground in the castle from the Queen, it was a coordinated attack. You and I know better than most that discretion is the better part of valor.