Good answers all around.
Something like this used to happen to me a while back. I used to masturbate quite a bit before I started hanging so every time my hands even brushed against my penis the blood would start flowing; having the bib on there sometimes made me erect. Something you’re doing while hanging is triggering an erection. Are you surfing porn or maybe subconsciously zoning out a bit and thinking about sex while you hang?
First, make like Para-Goomba said and ensure the hanger is on tight enough(the teeth need to be fully meshed). Then, try doing something to take your mind off the hanging. Read a book, watch tv or play a video game. If the problem persists in 30 days, bump this thread.