Whatever production process partially relies on arranging the crystal structure of the metal to arrive at desirable properties, the neutrons disrupt it.
Okay, I'll accept that: What used to be
the usual isotopes of iron, carbon, aluminum,
etc. with some extra neutrons, after
whatever gamma rays, alpha particles, etc.
boil off, becomes some other isotope or
element that doesn't fit in the crystal
and, thus, makes the crystal brittle.
It can happen without neutron absorption, the neutrons simply knock the atoms out of alignment, and they bounce around a bit (so each neutron can cause more than 1 defect).
(if it doesn't follow why that would matter, look into the heat treatment of steel)
http://en.wikipedia.org/wiki/Wigner_effect
Whatever production process partially relies on arranging the crystal structure of the metal to arrive at desirable properties, the neutrons disrupt it.