Reverse speech has the non tonal afterbreath placed first, making it sound like whisper.
Reverse speech is generally incomprehensible and sounds alien, so ends up in the general category of "fear of the unknown".
The reversed natural trail builds up momentary suspense. That applies to the reversing of most naturally fading sounds.
Audio for jump scares is usually designed to support what is happening on the screen at the time. This means that, without more information, anything here will be abstract at best.
In making a horror sound, there are several elements that you could think about using when you're designing your own.
Shrill noises tend to upset humans: fingernails on a ...
Adding on Michael's answer, there is an effect which makes you feel uncomfortable, but could be related to no.2. Like when a slow melody is played but the next note has no relation to the melody. It affects your brain like "Wow, wtf is going on here? This ain't right!". I think a reversing sound has a similar uncomfortable "WTF?!" effect, it's disjointed and ...
Sounds like a female voice, pitch-adjusted upwards for a synth-y quality, with a little fuzz for a buzzy static edge, a drop of phase shift for shimmer, and a short but noticeable time delay to introduce a second copy of the voice. The recording itself may be doubled instead of a delay; it's so short and heavily processed it's hard to tell.
It's the sampled scream that plays in the very beginning. It's pitched up, or rather played through a sampler instrument which would map the sample to multiple notes on the keyboard.
It does not sound like a synth, as some of the other answers have suggested.
An interview, a first hand account, and a strange treatise on the sound and history of animation and comedy. Go straight to the source and ...
The BBC did a cool documentary on this a while back which you can find here. They delve into questions of context, and well as cultural triggers.
Edit: This just appeared in my RSS... Specifically talking about game audio, but the science is the same.