You seem to be confused about "the situation causing her nervousness" and how that relates to the mentioning of her previous experience at a party. I really don't see how

But her reason actually refers to a totally different situation!

seems like strong evidence for the "cause" (I agree with what buybuydandavis says above about the use of the word in this situation) being the woman in question.

It seems obvious to me that she has beliefs based on her experience at the party that make the elevator situation worrying. This is not a difference in type from, say, one's beliefs about what a gun looks like informing one's nervousness in a situation in which one sees a gun, and I think it would be pretty silly to say that one's external situation was not the "cause" of one's nervousness in that case.