Some thoughts:

1. Reverse speech has the non tonal afterbreath placed first, making it sound like whisper.
2. Reverse speech is generally incomprehensible and sounds alien, so ends up in the general category of "fear of the unknown".
3. The reversed natural trail builds up momentary suspense.
4. Genre convention

Add those points up and you get something scary ;-)

Some thoughts:

1. Reverse speech has the non tonal afterbreath placed first, making it sound like whisper.
2. Reverse speech is generally incomprehensible and sounds alien, so ends up in the general category of "fear of the unknown".
3. The reversed natural trail builds up momentary suspense. That applies to the reversing of most naturally fading sounds.
4. The reverse build up creates a sense of something closing in on you.
5. Genre convention

Add those points up and you get something scary ;-)