I've often wondered this. (Do I have too much time on my hands? Maybe).

More answers at the source.

part of it is that the rotors create an enormous downdraft right down your neck. It's cold, that's why I duck.

With big helicopters the rotor blades can swing down a bit when they start slowing down. The tips of the rotor blades of a sea-king will be about chest hight for me when they are at rest. I'm not sure at what speed they droop that low though, as any movement will pull them up. And people are know to escape from those alive

Someone directed me to a site called Rotten.com (not recommended) and I got to see the results of a helicopter blade's contact with a human skull. It was too awful and haunted me for weeks. The top was simply sliced off like a boiled egg. I don't know how it happened - whether the helicopter tilted, but there it was in all it's awfulness. I instantly resolved if I ever had to embark or disembark from a chopper, I'd crawl on my belly.

I instantly resolved if I ever had to embark or disembark from a chopper, I'd crawl on my belly.

LOL, good idea.

S-c-a-r-y!