I think it's just a matter of what you are accustomed to. The girl who was afraid of bugs may have grown up in an area like Colorado, where bugs are not a noticable presence.

Growing up in a forest in Pennsylvania, I thought I was comfortable with insects. Until I saw the pterydactyls they call tree roaches that frolic in Tennessee.

I am not used to cities, and I am afraid of crossing busy streets. It may be the same case with that girl when it comes to insects.

Maybe it's background. If her parents had no tolerance of insects, she may have emulated them.
Just like any similar trait, it could be anything, and unless it's a dramatic/traumatic childhood incident, it's most likely nothing terribly interesting. :)

--Jez"it happens"zika