Hello,  I have seen several cracked plates over the years.  Most of them
were on old German pianos since they used thinner and less metal in the
frames than in the USA pianos.  Depending when the piano was made the
casting may also be porous or of poor quality steel plus other factors.

Sometimes it just happens, and I know of only two plates that cracked
while tuning.  It happened in our shop.  One cracked overnight, after
tuning, and the other while working on the piano.  It is one of the
worst sounds you can ever encounter.  We had been scratching our heads
at the time wondering why the plates cracked.  It could be from a sharp
knock or drop during a previous transport.  We never found out but it
was nothing particular that we did.

If the tuner raised the pitch to only A=435, then there is a reasonable
chance that the piano pitch was somewhat lower than that and the reason
for that is that the plate may have been cracked partially already
before he even started.  One would have to ask him.

The only other cause could possibly be that the pitch was maybe raised
really unevenly on a sort of note-by-note basis, starting on one end.
Are you going to repair this piano?

Best regards,
Bernt Damm
Sydney