I am giving a guess here, so feel free to take shots at this. I am assuming that the ん is の acting as a nominalizer. I will translate from literal to natural.
（１） こわいんです。 → こわいのです。
Scariness exists. → (Because) scariness exists. → "'Cuz it's scary." (The "it" here is assumed by me)
の here is a nominalizer, which perhaps we can think of as turning the adjective こわい into a noun= scariness.
Scary is because. → "Because I am scared." (assuming the subject is "I")
Technically, こわいですから may not be correct, though one does hear it colloquially. It might make more sense to say:
What is 〜からです and when is it used? How does it differ from 〜ですから and 〜んです?
Take further note that こわい is a tricky word to use here, because it can both describe the state of being frightened or something frightening. (*I am not sure of the validity of this claim :p)
Also, because of the lack of information I, as a reader, have on this conversation, I cannot accurately ascertain the subject or object being described (as would be necessary to accurately translate into English.)
I also think that どうして、とぶか is somewhat unnatural, especially if you wanted to say "Why'd you jump?" However, I am a novice at the Japanese language, so take my opinion with a grain of salt, especially if you found this conversation from a reliable source.