My understanding is that looner, is just a shortening of "loon" (as in "balloon") and the "er" prefix. I'm guessing that calling us "ballooners" seemed too technical, so someone decided to shorten-it to "looner" instead.
Just as you have "skater" in relation to people who skate, or "carer" for people who work in the caring profession. I don't think there's any more to "looner" than that, alas.