Was there a movie like Obsession in Japan at the time this was written? It's giving me similar vibes.
Why is Airi so scared? Because she's seeing things that shouldn't exist (dripping humans are not the norm 0_0) and feeling things she's not supposed to feel. Forced impetus has taken control of her and she doesn't know what's happening/has no control over it. That is, at least, the surface level answer I think.
However! What I'm curious about is whether that fear has a deeper layer to it. Is she afraid to be in romantic love with her best friend? If so, why?
Is she scared of ruining the status quo? Is she worried Nagi would think she's gross for liking the same sex? Does she think Nagi would feel like an object of lust if Nagi doesn't understand her feelings properly?
Falling in love can be a source of serious fear for people. You're losing control of your heart to someone else who may not even care about it. Someone who may not understand it. Someone who might even break it. Someone who you can't bear to be rejected by so you bury that love into the deepest trenches of your heart never to be seen by anyone, especially not THE ONE.
Is that what happened with Airi? Did she bury those feelings and the hypnosis brought them to the surface?
So love and fear being blurred together in her brain while in a fugue state could have some real purpose to it.
I'd say this is a fun way to briefly delve into the horror genre without just being an "XD so random," kind of deal. That is, If this is a springboard for deepening the relationship between Nagi and Airi.
Either way it's fun, way more fun to ponder about than I would have expected. I guess I like the genre after seeing the movie Obsession. I'm hoping there's more purpose to this genre and tone shift than just to be funny and unpredictable.