String breakage

[Newton Hunt]

Metal fatigue, pure and simple.  Piano wire can be bent only so many
times before it breaks.  It matters not when these bends occur, 1 second
apart or twenty years apart, it will break with the same number of
bends.  (I just checked with a friend who is a metallurgist).  Your wire
broke most likely at the tuning pin.  This is an area of frequent
flexing while tuning and is therefore, subject to frequent fatigue.  One
of those things we all must endure.

        Newton