Surface tension favors forming spheres from condensing materials in the absence of other forces. Soap bubbles are spheres for the same reason, absent winds. See Shot tower - Wikipedia for more about a manufacturing process that relies on this effect.
As the vaporized metallic & mineral stuff in a nuclear fireball is expanding and cooling, it isn’t being thrust through the atmosphere. Instead the local atmosphere is being thrust along at more or less the same rate. Absent much net local wind, the vapor coalesces into spheres, then cools to a solid.
Note that tektites – naturally-occurring glass formed during high-energy meteorite impacts – often have rounded or even spherical shapes.
LSL and Snail: yes, i figured it was the standard fluid dynamics. Just interested if shaped-charge-like effects also were involved. Spalling I doubt.
Nuclear S’mores. Guaranteed to brighten anyone’s day.