Fuuuuuuck... she is possessed by a nonagare, it is using her to infiltrate Dara-san's territory.
Dara is strong, but she has pointed out that she has limits and weaknesses. Nonagare seem to be a category of youkai that are actively trying to take territory from local deities, and as such, to receive that definition, they have to be strong... and smart.
It must have felt the kin bond Chihaya has to a yamagami, or even... it could have been already planning this, keeping tabs for a chance to start its assault.
Away from the defenses of Dara's territory, Nagi and her "uncle" being spiritually blind was a golden chance... and it took it.