The trouble with the 100% yield version that was originally planned is it would have dumped a lot of radioactive fallout on Russia. The 50% version was not so much dialled back as a different design with some of the uranium components replaced with lead.
Thank goodness. I'm also glad they tested it high enough up to prevent the fireball from touching the ground. With a modern bomb you get fallout from the primary stage but the vast majority from an air burst is going to be from the outer uranium-238 shell which absorbs all the extra neutrons from the fusion and splits without emitting any neutrons itself. This isn't a problem with lead for reasons I don't understand and with the air near the bomb you're just doing stuff like turning nitrogen into carbon-14. The results either decay immediately or last for a thousand years and so aren't particularly radioactive. If a bomb goes off near the ground or sea water then there's plenty of stuff possible products with those nasty intermediate half-lives.
Well, lead is a very stable element from what I understand. Radioactive elements like Uranium are radioactive because they're inherently unstable, such that fission is 'easily' doable and releases a lot of surplus energy.
Looking it up in a reference, natural lead is about 1% Pb-204, 24% Pb-206, 22% Pb-207, and 53% Pb-208. These are all of the stable isotopes.
If Pb-204 absorbs a neutron it turns into Pb-205, which isn't stable but its half life is ten million years so it's only very weakly radioactive. The -206 and -207 turn into other stable isotopes, of course. So those don't matter so much: about half of your neutrons enter lead nuclei which can safely absorb a neutron.
The other half, the Pb-208, turns into Pb-209. Pb-209 is radioactive with a surprisingly short half-life of 3.25 hours, so that's gone in a matter of weeks.
The nasty stuff is the lead atoms which happen to absorb two different neutrons during the explosion. This becomes Pb-210, which has a half life of 22 years: short enough that it's going to be emitting a bunch of radiation, long enough that it won't be gone after a few weeks.
So the answer is probably "it's very rare in these sorts of explosions to have two neutrons hitting a single lead nucleus." There are other possibilities (maybe they got the lead from spent nuclear fuel or centrifuges or something and it's all Pb-206) but that sounds like it's the most plausible.
It would also have been far more terrible even in the 50% yield if it had been let to touch the ground for the energy to turn into seismic waves. The energy corresponded to a 8.1 on the Richter scale.
my head cannot fathom what 100% yield would look like.