what is the principal cause of ichabods defeat?

Ichabod’s imagination leads to his downfall in two ways. First, his imagination leads him to get completely carried away about the situation with Katrina, such that he thinks his chances are much better than they are, and he fantasizes about the future so much that he cannot imagine failing--but this also keeps him from making the necessary life changes to become the kind of person suitable for her. Second, his great enjoyment of ghost stories and other supernatural tales, which he actually believes because of his strong imagination, makes him utterly susceptible to Brom Bones’s prank, so he is doubly defeated.