It has happened to me aswell. Basically if everyone else quits or gets disconnected from the race, apart from you, when you are the host, the race will keep going until the end. However it doesn't reward you for it, so you may aswell quit it. I once had a race with me and four others, they had all quit by the time I finished lap 2. Luckily this doesn't happen very often, but it sure is frustrating when it does.