Why?
Presumably they volunteered, on the offer of some trivial inducement.
They weren't so badly off, being about 4000m away; as long as none of the crud came down, but drifted up and away.
I imagine the main risks were rocket and safety (safety? In 1957?) failure, leading to the warhead going off rather nearer than they expected, or some fallout catching them at the time or later.