I am guessing its what killing off 9 regions cultivators does, it removes possible composition in the 9 regions.
So it could be a group from the 9 Regions doing it to weaken the other sects to improve their own comparative power within the region.
And due to how long it can take people to develop their cultivation, if a sect can deplete others sects low - mid level cultivators [higher ones being more likely not to be vulnerable to the scroll tactic as pointed out in past chapters] Then they could get a numbers advantage in the future.