I don't get it either but I think it's just an expression of exuberance. Like "hooray, did you see that? What a wonderful world!"
When we had a sailboat, my buddy and I were grabbing a mooring on Martha's Vineyard when these girls on another boat whooped at us and flashed their breasts. It was weird because their parents were just below in the cabin. Only time that's happened to me.
Man, I've seen that in movies, though never here. Maybe it's an American thing :)) I think it's different though, or at least, I imagine it to be - an act of rebellion, a 'wild' moment while remaining completely safe... which I suppose is the same as what these men do, except they probably do it more often.
What a wonderful world where we can honk at random weird :))