I agree that some people are letting you know that they are there.

BTW...I grew up on Long Island and when I was learning to drive (admittedly a long, long time ago) New York State Law mandated that a car overtaking a bicycle was required to honk and warn the cyclist that a car was approaching. I'll bet some states still have some sort of similar law that is simply not enforced and not widely known about.

As for the cyclists above who "move to the left" when honked at...Do you really think it's wise to intentionally move into a lane in front of a car on purpose???