Why are people tapping their horns when they pass me?

I would say there is a very low percentage of times this has happened to me, but nevertheless, I am growing interested to find out why it happens. I would like to know if any of you honk at people walking (well out of the way of passing cars) down the road, and if so, why? I would like to be clear on the messages I have been receiving through people honking when they pass me. Thanks.