...at the drop of a hat

"What is the etymolog of that idiom?"
As I understand it, this is meant to be saying that someone will do something very commonly, ofr almost no reason. Well, the almost no reason seems right, as there's nothing significant about hat dropping (although I think dueling gloves aren't all that significant either, and dropping hats could nearly as easily have been a similar social custom, just that, as far as I know, it wasn't). There's a much bigger issue I have with the phrase in question, though. Who drops hats? I don't think I've everseen this happen, and so I wonder why this particular strange event was chosen for the idiom.