I'm glad that it hasn't happened to me, but if you consider the amount
of stress that a piano frame supports it seems to me amazing that it
doesn't happen more often.

I did a quick search on the internet and found reported that there
is a cumulative 20 tons of stress across the frame.  When the piano is
properly tuned, it is balanced and the frame can support the stress.
When replacing strings on the piano (which I have never done myself)
you must release the tension slowly across the entire scale alternating
strings in order to keep the stress levels balanced, the same would be
for bringing the piano up to pitch, and I have seen the effects of a
piano that was released unevenly until the frame cracked -- destroying
the piano.

From what I read on this string, it sounds like this particular case
the work was done correctly so one can assume that it was an accident
waiting to happen.  Years of stress tension can take its toll, and
there are also pianos that are inherently designed poorly.  (Bluthner
pianos from the turn of the previous century have this problem and when
buying a Bluthner, one should carefully inspect the frame for stress
marks).

Will a repair hold?  Likely not.  There are several techniques that
have been tried: welding, a cross bridge, etc.  Sometimes they hold for
a while, but they will always have a stress point and likely will not
survive the test of time.  I believe that it's a case of another one
bites the dust.

Best regards,
Eliyahu Shahar