It is possible for a plate to crack whilst tuning, especially if the
pitch was much lower than normal.  An additional two tons tension will
possibly be added to the plate; this strain is taken up by the plate
with influences from the wood from which it is secured.  The casting
may have not been good when made -- the faults hidden in manufacture,
but not deliberate -- and this may be the cause of the cracking.  There
is around 35,000 to 40,000 pounds tension on a large upright piano.

Stephen Bentley
Vancouver, British Columbia, Canada