At one time, I was running three different weekly quizzes in three different pubs in my locality. When I returned to my local one evening after presenting one, one of my friends said, "Ah, here's the monster quizzer!"
The nickname stuck. However, when I decided to join AnswerBank, it allowed only a maximum of eleven letters in user-names, so that one was rejected. I made as small an alteration of it as I could and came up with Quizmonster.