the ploughman may Have heard the splash, the forsaken cry, But for him it was not an important failure; (14-16)
The "forsaken" nature of this fall is really what changes an unfortunate accident into outright suffering. Too often, it seems, other peoples' attention can turn a nasty accident into a lucky break – or not.
the white legs disappearing into the green Water; (17-18)
There's a certain casualness to this description – even though we'll eventually get the whole story, Auden's speaker treats it with the same nonchalance that he imagines for the other figures in the painting.
Something amazing, a boy falling out of the sky, (19)
Yup. That's bad, all right. But notice how Auden always tempers his descriptions of awful things with strange adjectives? They make disaster seem spectacular – if not outright exciting. Which is why we say that there's a healthy dose of irony in his perspective, come to think of it.