Maybe you told him you weren't interested in a relationship. It sounds like he had an inkling that what you meant was that you did not consider him to be someone worthy of having a relationship with you. Sure you were honest, to a point. Most people are going to catch feelings when they realize their role is only to fill the space of someone else's future girl/boyfriend but they have personally been judged as not good enough for that role.

People catch feelings when they have to train a replacement or someone with less seniority for a promotion they feel like they earned.

Maybe you feel like you were honest enough, but perhaps the position you were asking him to fill necessarily appealed to someone with self-esteem issues and some self-deprecating behavior. It's like you want to be treated like a queen by someone who deep down you don't feel is completely worthy of your time. People in general just don't react well to being treated like bookmarks or entertainment.