I dunno, Q. My grandparents, who'd be about 120 if they were still alive, used "rounder" solely to mean a guy with disreputable social habits like drinking, gambling, and especially hanging around with "fast wimmin". They didn't know why it was "rounder" any more than they could explain the more thuggish "loafer." It was just one of those things.

The connection with "roundhouse" comes, I think, from the opening of the Newton & Siebert version of "Casey Jones": "Come all you rounders." Supposedly Casey's widow was very concerned that the song would tarnish his memory by hinting that he was a "rounder" too. (Not to mention the part where she tells her kids they've "got another poppa on the Salt Lake Line"!)