This is because a helicopter, and of course anything metal attached to it, has a tendency to develop ONE MOTHER of a static electricity charge, what with that rotating air-static-collector thingy they fly with, and all.
If somebody were to just *grab* the basket that comes down from a helo they'd get a static shock like nobuddy's bidness. If it has been hanging there for a while (some minutes), the built up static charge could even be life-threatening.
That's why before lowering it into a boat or onto something, they always try to dip it in the water first, or let it touch ground, to dissipate the static charge.
You know, I'd dearly love to ride in one of those choppers...but I'd just soon not get in one that way.
I noticed that, but thought they had just overshot the guy on the roof because of the wind. But now that you mention it, I have heard of that before. I remember that scene in Hunt for Red October when they're lowering Jack Ryan to the Dallas, and they're yelling not to touch him.