Not eggs, but prey. They use this movement to break its bones and kill it.

It’s a siriema if I’m not mistaken. There are a bunch of them in the country side of where I live (Brazilian Cerrado). They are absolutely fantastic animals

Ayup. These birds make the same movement with snakes and other animals to break all their bones.

I assume the bird’s knowledge is inarticulate and all it knows is that it’s very satisfying to slam the ball on the concrete.