Just curious measure the length of the two rackets you think went well. Are they shorter than they should have been? If they are are not right your standards are slipping for sure. Just because they didn't break does not mean they won't or all went well. If they are the correct length the standards could have slipped on the other racket (because probably you had to adjust them and the pressure of the mains pulling on the top and bottom of the racket broke the racket because there was no countering force from the sides of the racket.
It is better to complete the racket all at once. Never start a string job unless you are sure you have time to finish.
I would like to apologize to anyone I have not offended. Please be patient I will get to you shortly.