That's most likely. [She ponders a little longer, walking towards the rift again.]

So we have unknown creatures and rogue Servants... If they're coming from the same source, we're dealing with two different beings that are starved for prana.

[Ai Thao looks at her unused Command Spells grimly.]

Perhaps, in the Servants' case there are few that somehow get through the rifts, but encounter no Master to contract them... And whatever essence is in the cracks is driving them mad, but gives them enough energy to hunt?

It's just a theory. But a dire one nonetheless.