He is radiating confidence so why are they mad?

There is this guy on my job who has always been made to feel like he is a worthless piece of trash by some of our co-workers and managers. Recently I have noticed a change in his demeanor. He has been appearing more confident and hopeful than usual. This has caused some to step up their antics and it does not seem to be working. One co-worker even told him "what so you think you are superior to us now" and he confidently replied "no, I do not think I am superior to anyone nor do I think I am inferior to anyone" then he smiled and left. I must say that I am loving his new persona. But why are some co-workers mad at him for that?