I had a racket with a lot of wear into the frame break at the head after I strung. I have a Stringway machine with a very good 5 point lock down mount system. The racket was a lighter wide body that was over 2 years old of heavy play - +10 hours per week for 2 years. I think the frame was weakened by the wear and the pressure of new strings cause it to crack at the 10 o'c position. It was an over sized Fischer strung at 54 lbs with a multi-filament.
I have never had another frame do this in 6 years or so of stringing.