It seems like this happens to guys who wear briefs or some other supportive underwear. The same thing used to happen to me when I would occasionally wear boxers instead of briefs. 2 years ago, I started to dislike wearing snug underwear. Finally, I said screw it and quit wearing briefs. My only concern was at work. Everything would be cool if I was sitting down, but if I had to walk through the building, feeling the sensation of my dick moving around and rubbing against my jeans would get me rock hard. After 2 - 3 weeks, the constant erections subsided, but they still happen every now and then. Once you get used to hanging loose, you can't go back.