The article I read it in (in independent Polish press, cannot find it now) said physical exertion makes you more susceptible to radiation, and cited figures around the weight of shells which are apparently quite heavy.

It doesn't, the autoloader is right in the crew compartment (which is the whole problem with munition cook offs killing the crew and ejecting the turret). The thing that keeps NBC out while the breech is open for loading is positive air pressure inside the tank.

This argument only makes sense if the turret is entirely unmanned.

Possibly that the higher number of crew & higher average effort means the internal resources can not last as long, and thus the NBC seal has to be broken sooner?

According to the wiki, an autoloader also improves smoke removal, which would also help maintain a safe NBC-isolated interior space.

Maybe the turret has less shielding..? Or maybe it was really to enable DU rounds.