I don't think that her crush ever had anything to do with it. I think it was the letter and the circumstances more than anything. The letter made her view Robbie differently then before. Then when something terrible happened, she thought of the letter, and her mind linked the two together, leading her to think it was Robbie, when it wasn't really. She probably supressed the memory of who it actually was until she was old enough to realize the contents of the letter didn't mean Robbie was capable of rape.