There is always the possibility of a defective casting.  As everyone
here knows, metal alloys are ferociously complex assemblages of
crystals, and a poorly alloyed, improperly-cooled iron casting can
be treacherous.  I'm not entirely sure of this, but I believe I've
read that there's even a certain degree of chemical reaction in the
solidified iron long after the casting has cooled, where improper
admixtures like sulfur and phosphorus can weaken crystal boundaries.

There's also our old friend stress corrosion, where a part made of an
alloy that's normally fairly passive will exhibit considerable chemical
reactivity at portions that are under deflection.

And so I imagine that the catastrophe, which is very much what it was,
may well have been the fault of whoever was supposed to be supervising
the guys who threw five old window sash weights and a bedspring into
the cupola furnace at the foundry where they cast that plate.  I would
imagine that piano manufacturers, who don't need this sort of thing,
try to keep an eye on their foundries, but my understanding is that
plates in the US are made by only one outfit.

Mark Kinsler
Lancaster, Ohio, USA
http://www.mkinsler.com/
http://howthingsoughtawork.blogspot.com/